First off you need to read up a bit on battery charging algorithms. Know that sealed lead acid, NiMH, and Li-Ion are charged in completely different ways.
All of these are damaged by overcharging. "Trickle charging" does not get around the problem, it just damages it slower but it'll still be damaged and usually the "trickle" part means people intent to leave them on the battery for days/weeks/months past the time it took to fully charge them.
NiMH is often charged by applying a constant current and then looking for a temporary drop in the voltage that occurs around a full charge. It's easy to miss so there are other detection methods involved, like for one if it's an AA cell it should definitely just shut off if it's already given it 2500mah.
There's a LOT out there on charging the tricky batteries (NiMH/Li-Ion/LiPo). Some of it is proprietary or sort of an "industry secret" though.
And the short answer here is, "can I pick up an unknown NiMH battery and check its charge state?" the answer is there's no practical, reliable way. Unless it's empty of course, that's easy to see. If you want to determine how far along it is in the charging process at any point, also not very reliable. You can only figure out when it's through, and then only with some particular agorithms.