Could be from over tightening it or leaving it too loose. Too tight= stretches the chain prematurely. Too loose= jumping teeth on the sprockets.
Or it wasnít timed correctly in the first place. As you install the exhaust cam, all slack has to be removed from the chain at the front of the engine while keeping all of the marks lined up. Then, do the same with the intake cam. If you donít, then everything is off by a tooth or more after you install the tensioner. Not saying either is your problem, but both can be contributing.
There are a few ways to tell if you bent a valve:
Compression/ leak down test is the most reliable.
You could pour a little gas into each valve pocket. If it disappears, the valve isnít sealing. Could be other things, like a cracked head, stuck guide, etc, but not likely.
Check valve clearance. If itís over the limit (too big), then the valve isnít closing all the way (bent).
Personally, I figure if there is damage, itís already done. Re-time it, turn it over by hand to check for contact, do a compression test.