Yes, we should trust Colin Powell for what he says about Isreal. When a former Secretay of State, Chairman of the Joint Chiefs of Staff, National Security Advisor and retired four star general, et al, says something about foreign relations, we should believe him. I was not a fan of the administration he worked with, but I respect his accomplshments and know that he is a highly intelligent man. He may be exaggerating a little but this man has access to knowledge that not many people on this Earth have, so his credibility is rather solid.
I don't see what Colin Powell's agenda would be in fabricating information. He's not running for office, or campaigning for anyone who currently is, so what's his ultimate goal in saying that? At the time he said it, he was Secretary of State and in a position to have some intelligence information about it, so he's probably not completely wrong.
Yes, we can trust Colin Powell. Colin Powell is a trustworthy figure, first of all. Second of all, I would not be surprised at all if Israel did in fact have 200 nukes. The US has the most nukes in the world, and Israel is one of our closest allies. They are also in a violent hotbed in the world, so it would make sense for them to have a lot of nukes.
No, we should not trust Colin Powell when he says Israel has 200 nukes until he provides the proof. What are his sources? Where did he hear this? He should be willing to cite his sources in order for us to trust him. It would be uninformed to blindly trust him.