Is there a way I can check where my RAM is currently allocated to / being used for?
And how can I re-use it for other things?
Unfortunately, as of now, there is no easy method of achieving this. One must check all actions sent in the past and look up if any of the contracts that they interacted with use their RAM. To free that RAM, one has to check the ABI of the contract to see if the contract provides any way of freeing up the RAM. Unless they do that, there is no way to free the RAM at the moment.
On the bright side, this service is something that the community can create with the current tech, so it may be just a matter of time until a web app can let you know where your RAM is allocated.
This question has also been asked here, but it doesn't have any answers either.