How do we know Paul was a true Apostle? Well, he called himself one in many verses, for example:
1 Timothy 1:1 "Paul, an apostle of Christ Jesus according to the commandment of God our Saviour, and Christ Jesus our hope" (ASV)
1 Corinthians 9:2 "If to others I am not an apostle, at least I am to you, for you are the seal of my apostleship in the Lord" (ESV)
Romans 1:1 "Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother," (ASV)
There are many other passages where Paul refers to himself as an Apostle, but the main reason we can know he truly was one, is because Scripture says he was called and commissioned by יהוה (YHWH) Himself!
Acts 9:1-22, specifically verse 15, says: "But the Lord said to him, 'Go, for he [Paul] is a chosen instrument of mine to carry my name before the Gentiles and kings and the children of Israel'." (ESV).
Let’s take a look at this entire passage in the Aramaic:
Acts 9: 1. Now Shaul was yet full of intimidation and the fury of murder towards the disciples of our Master. 2. And he asked for letters from the chief priests to give to Darmsuq to the assemblies that if he should find men or women who follow in this path he might arrest (them and) bring them to Urishlim. 3. And as he traveled and approached Darmsuq, suddenly a light from heaven shone upon him.
4. And he was thrown upon the ground, and he heard a voice that said to him, Shaul! Shaul! Why do you persecute me? It is hard for you to kick the stakes. 5. And he answered and said, “Who are you my Master?” And our Master said, I am Y’shua the Nasraya, He whom you persecute! 6. But rise up (and) enter into the city, and there will be spoken to you concerning what you should do.
7. Now the men who were traveling with him on the road were amazed because they heard only a sound, but a man was not visible to them. 8. Shaul got up from the ground and he was not able to see anything though his eyes were open. They brought him by his hands to Darmsuq. 9. And he was not able to see (for) three days. And he did not eat and he did not drink.
10. Now there was a certain disciple in Darmsuq whose name was Khanan-Yah. And Master YHWH said to him in a vision, Khanan-Yah! And he said, “Behold, (it is) I  my Master.” 11. And our Master (Y’shua) said to him, Arise. Go to the street that is called Straight and inquire at the house of Yehuda for Shaul who is from the city of Tarsus. For behold while he was praying 12. he saw a man in a vision whose name (was) Khanan-Yah  who entered and placed a hand upon him so that his eyes might be opened.
13. And Khanan-Yah said, “My Master (Y’shua), I have heard from many concerning this man how much evil he has inflicted on your Set Apart ones in Urishlim. 14. And behold even here he has authority from the high priests to arrest all those who call (upon) your Name!” 15. Then Master YHWH  said to him, Arise! Go because he (is) a vessel to me, chosen to carry my Name among the Gentiles and among kings and among the sons of Israel. 16. For I will reveal to him how much he will suffer because of my Name.  (AENT)
 Notice how Khanan-Yah is reproved not by Y’shua, but by YHWH Himself, as indicated by the Aramaic word MarYah. This represents a profound dichotomy because Khanan-Yah himself may not even be aware that the Father is responding to him. Khanan-Yah’s point is that those who proclaim YHWH’s Mashiyach are being persecuted because they “call upon your Name.” But YHWH points out that it is His Name, in Mashiyach that is being rejected, so when all is said and done, Shaul is preaching YHWH’s Torah (instructions) to the Gentiles.
 Aramaic here presents a critical distinction lacking in Greek, once again due to the bifurcation of MarYah (YHWH) and Maran/Mari (Y’shua). In verse 5 of Acts 9, Y’shua gives a command that is questioned. This act then prompts the speaker to switch from Y’shua to YHWH; therefore, it is YHWH’s name that Paul will suffer for!
It is also important to note that while Y’shua was on earth, any teaching about Y’shua being YHWH refers to the divine nature within Him. Y’shua was both conceived through the Ruach haKodesh and He breathed the Set-Apart Ruach into His disciples. Here, the resurrected Y’shua speaks to Paul in the vision. Y’shua’s human nefesh reanimated with his Ruach in a spiritual resurrection body, as described by Paul in 1 Corinthians 15.
 Ha ana is the Aramaic equivalent of hineni, the famous Hebrew phrase uttered by Awraham when called by YHWH, “behold it is I” or “here am I”. Awraham responds to YHWH during a time of testing, similarly Khanan-Yah is being tested when asked to give aid to someone who could imprison him or even kill him for his faith. Being asked to go to the street called “Straight” is a major Hebraic idiom, as we are to walk the straight street when we live according to YHWH’s Torah.
 The name “Khanan-Yah” means “Yah is merciful.” The play on his name demonstrates the mercy about to be shown to Shaul through the restoration of his eyesight. This is only apparent in Aramaic.