Standing up for you as in protecting you? Sorry, I got a little confused as to what you really meant.

Anyways, if what I said is what he's doing, I think that it'd be best if you were to just ask him yourself. Spying definitely wouldn't do any good, as you should trust in him. Even if he's done something to break that trust, I wouldn't go around spying and getting in his business. If you really can't ask him yourself, I suppose you could ask some others, but you would REALLY get more from just talking to him.

If it's not the protecting kind of "standing up", as in it means he stood you up for someone or something, I still think that you should have a conversation with him. It could help to some extent to ask others what's going on, sure, but sometimes they don't tell you the right information. I would imagine it being one of those "he said, she said" things that would keep you running in circles. I personally wouldn't want to bother with a wild goose chase or however you would like to put it.