Any fish I catch (and it wasn't many this year

) hooked themselves and the strike is just to drive the hook home and make sure it didn't come out make sure that the hooks are as sharp as possible this makes everything alot easier.
small taps could be crabs or small fish nibbling at the bait the only way to find out is to put a really small hook on and see if you catch one.
if you are fishing from a pier it could also be your weight being pulled by the tide

just bobbing up and down.
its a bit mad but the best way to learn is to catch loads of fish you eventually get a sense of when the fish is hooked and when is a good time to strike.
Flatties I just let them hook themselves you see the one definite nod on the rod and leave it for a few minutes.
doggies a lazy pull more than a crab try and get them before the swallow the hook.
conger can be tentative to start and then just go nuts strike quickly
Bass just go nuts I always strike as quickly as possible.