Originally Posted by nelly
You'll never be the only idiot in the suit because:
1) Someone else will always wear a suit
2) You can always take your jacket and tie off
That being said, I think the simplest solution is to just ask the person you know that's getting married. It's not like it's an uncomfortable/awkward question. Just shoot them an e-mail.
I was in this situation before, and I wore a suit but took an odd jacket and a pair of chinos just in case. Also brought a button down collar in case I wanted to switch to smart casual. Arrived at the wedding, most were dressed smart casual. Saw a couple of suits, so figured with no loss of generality I was good to go. Wore the suit with the tie and pocket square and was happy as a clam.