Should I use fake profile to see if my bf would flirt with another girl?

Sure, if you want to be caught lying and snooping, and destroy your relationship in the process, regardless of this guy's innocence or guilt in the matter. How awesome would THAT be?

Why not talk to him about your concerns?

If you can't talk to him about stuff like this, there's no point in continuing on with him in any case.

If you don't trust your boyfriend break up with him.

I think it's possible you have an over inflated estimate of your BF's appeal. Fix that and all the other things might fall in place.

You don't trust him so you want to be devious and hire someone to flirt with him to see if he bites? Wouldn't that lead him to think that you are a bit untrustworthy? I mean, if it were me and I were to find out that you did this, I would end things very quickly.

This is exactly what communication is for. You sit down, you talk to him about it. Why are you suddenly not trusting him? What has happened that caused you to not trust him much lately? Talk to him about it. A relationship where one party tests the other party is not a relationship worth having.

same here as everybody saying. This sounds nasty. If you have doubts, talk to him.

I think we can all see who the untrustworthy person is here, and it ain't him.