If a cell is failing and has an internal short you will find that the battery charger is working much harder (higher current) and for much longer that previously as it is, in effect, trying to put a charge in faster than it is 'leaking' (shorting) out,
The acid will be boiling away, the battery case will start to get warm, the battery terminals will get too hot too touch and the battery case ends will start to bow outwards.
The 'fumes' will start to corrode any metal in the vicinity and you may notice a 'greenish' tinge to the metal work, or even the start of a crumbly white powder coating (I'm sure you've seen it on old battery terminals).
The longer you leave it the worse it will get - the final indicator of an internal short is when the battery actually explodes spraying hot acid all around the engine room, and, onto anyone who is standing / sitting / working in the area.
Once you have identified (and accepted) you have a problem it can become quite dangerous, and very expensive to delay taking the battery out of the bank.
Not necessarily to replace it but just make sure you disconect it.