In the first-century testimony of Christ was more accessible. Eyewitnesses of Jesus were available to whom you could go. I am convinced that virtually all of Israel, around 2 million people in that time, witnessed Jesus with their own eyes. It was easier then because most people from that region knew someone who had been healed or seen one of Jesus' miracles. Testing the authenticity of Jesus was as easy as asking a friend or acquaintance what they saw.

Now things are much more complicated. Although, The testimonies of eyewitnesses were written down and collected into the New Testament. However, no one who saw Jesus is still alive. So, we have to rely on the words of strangers from the past. That does not mean that those words are not verifiable. There is an incredibly potent way to test Jesus' authenticity. The woman at the well said to Him: 

“Sir, I perceive that You are a prophet.” (John 4:19) 

She was right. Jesus made several prophecies. This is how we can test the reliability of Jesus's words today. It's simple. Deuteronomy told us how to do it:

You may say to yourselves, “How can we know when a message has not been spoken by the Lord?” If what a prophet proclaims in the name of the Lord does not take place or come true, that is a message the Lord has not spoken. (Deuteronomy 18:21-22)

So if what Jesus prophesied does not come true, He is not a prophet. If He's not a prophet, then we ought not believe in Him. In our attempts to authenticate Jesus, let's take a look at some of the prophecies that he made. One of the most far-reaching is this:

When you see Jerusalem being surrounded by armies, you will know that its desolation is near. (Luke 21:20)

During a chat with His disciples about what was to come, Jesus made a few comments that are astounding in their scope and accuracy. Jesus prophesied that Jerusalem would soon be surrounded by armies. (Luke 21:20). This was fulfilled in the lead up to the destruction of Jerusalem in early 70 A.D. The Roman army built siege ramps around Jerusalem's walls before they were ultimately breached. Here is another worth looking at:

The days will come upon you when your enemies will build an embankment against you and encircle you and hem you in on every side. They will dash you to the ground, you and the children within your walls. They will not leave one stone on another…. (Luke 19:43-44)

Jesus not only prophesied that the city would be the victim of siege, and have embankments built around it, but also that the people who didn't flee would be trapped inside with their children. This prophecy came true a few days before Passover of 70 A.D. It took three weeks for the Roman Commander Titus to breach the outer walls. It’s valuable to note that Jesus was long gone by the time this prophecy came true. Another such statement from Christ comes in Mark’s account:

And as he came out of the temple, one of his disciples said to him, “Look, Teacher, what wonderful stones and what wonderful buildings!”  And Jesus said to him, “Do you see these great buildings? There will not be left here one stone upon another that will not be thrown down.” (Mark 13:1-3)

Jesus prophesied Jerusalem would be destroyed; This was also fulfilled in 70 A.D. After breaching the walls, the destruction and murder of millions began. Jesus also foretold that the buildings in Jerusalem, including the 2nd Jewish temple, would be destroyed. This was fulfilled in May of 70 A.D. After the rest of the city was conquered the last vestige of resistance holed up in the temple. The Roman conqueror, Titus, didn't want to destroy the temple but instead convert it to a temple for the pantheon. However, by fire it was utterly destroyed, Just as Jesus predicted. On a similar subject Jesus said:

“How dreadful it will be in those days for pregnant women and nursing mothers! There will be great distress in the land and wrath against this people. They will fall by the sword and will be taken as prisoners to all the nations.” (Luke 21:23)

Jesus predicted that after this event Jews would be scattered throughout the World. It happened as He said it would. Those who were not killed fled in every direction and took up residence in all parts of the world. Jesus prophesied that Jerusalem would then be controlled by Gentiles. (Luke 21:24) After the destruction of Jerusalem, the city was controlled by Roman, Byzantine, Persian, Arab, Christian Crusaders, Mamluk, Ottoman, and British rulers. Gentiles ruled over Israel for around 1900 years.

Jesus prophesied persecutions of the Jews who were scattered. (Luke 23:28-30, 21:24) This has been fulfilled over and over in every generation for the last 20 centuries as represented by the Holocaust, pogroms, and other persecutions against the Jews. This is significant in history because no people group has ever been pursued by persecution like the Jews. It goes beyond reason. The persecution of the Jews requires a supernatural explanation within itself. It would be the wildest of conspiracy theories... If it weren’t true. The most natural outcome would be that the Jews should have died out long ago, but Jesus prophecies that they would continue to live, and thus they have. 

Jesus and the apostles John and Paul predicted that despite all these impossible circumstances, the Jewish people would survive (Luke 21:24, Revelation 7 & 14) This is significant in history because virtually all other ethnicities of any historical era have ceased to exist by assimilation. Virtually all are eventually mixed into other groups. The Jews, however, have survived and stayed distinct despite the fact that many nations have evicted, persecuted and tried to eradicate them. Nonetheless, Jesus said they'd survive, and that is precisely what they have done. Jesus made an incredible prophecy:

Jerusalem will be trampled on by the Gentiles until the times of the Gentiles are fulfilled. (Luke 21:24)

In this Jesus predicted a number of details that no natural man could know. After being scattered for a time, Jews would return to Israel. This prophecy was incredibly fulfilled in the 1900s. The most notable event of this return was in 1948 after WWII and the holocaust when Israel was established once more as a homeland for the Jewish people. This is utterly unique in all of history. 

Secondly, Jesus predicted that the city of Jerusalem would come back under the control of the Jews. John also implicitly prophesied the same. This prophecy was fulfilled in 1967 after the six-day war. It's not only incredible that He predicted this, but it's incredible in the history of the world. No other people has ever been driven from their land and returned generations later to gain national sovereignty. The Jews have done it—not once but multiple times.

There is no way that a first-century person could have known that Jerusalem would be conquered, and ruled by Gentiles for a time while the Jews were being persecuted throughout the world, only to return and regain national sovereignty. This is nothing short of a miracle that can be authenticated in the modern world. If Jesus has the power to predict the future, then He is trustworthy.