Fermenting.. How to check for air leaks?

[jackfiasco]

So I fermented a batch of sweet feed in a 5 gallon sea salt bucket I got from a local restaurant and it worked out fine, airlock bubbled away for close to a week, then stopped and cleared. I wanted to do bigger batches so I got a 6.5 gallon bucket from a local brew shop and also a 25L bucket from Brewhaus that came with the PSII I bought. I started fermenting sweet feed batches in both of those buckets and they didn't bubble as much as the first batch and they seemed to stop in just a few days.. I let them sit for a couple days more and opened them up today to see that they aren't cleared and are still bubbling inside, so there must be an air leak somewhere as the airlocks aren't bubbling at all. Also one of the airlocks is full of dead fruit flies - I didn't realize the holes in the airlock lid were big enough for them to get in! It seems odd that both of these different buckets, that are made for fermenting, wouldn't be airtight - when my used sea salt bucket was - though the sea salt bucket's lid has a gasket in it and the other two don't.

Is there anything I can do to check for and fix the leaks.. like add a gasket to the lids or get different/better lids or something?