I have switched suppliers a few times, lately i must admit i've been too busy to look at it but my bill is so low i'm not sure i'd get a better rate elsewhere anyway.
You should get a regular bill/statement, on this it should say if you are in credit or debit.
Technically you should be in credit over the summer and this is eaten up in the winter when the heating is on, so you should be in credit.
I can actually submit manual readings at any time and at that point i get a snap shot of how my account is looking, not sure if you can do the same as this would be ideal to see if you are owed anything back if you decide to leave.
I think that energy suppliers have been burnt (pardon the pun) by OFGEM so much that they daren't not give you an accurate credit when you leave.