How to ensure you have a fair acting contract
There are a couple of ways of making sure that you get a fair acting job.
Firstly, having an agent would be the easier way to get fair contracts, your agent should be there not only to help you getting jobs, but he/she should also make sure that you get the best deal - it's in their best interest, so you should trust them.
Secondly, you could check the average ammounts that companies pay for a specific type of role, for instance, non-speaking roles are obviously not as well paid as speaking roles. In tv adverts, everything counts: if you have your back to the camera you will get an ammount, if your face shows then you will get more money. If you only say a line, you will get more than someone who does not speak, but you will get less than someone who speaks more than you, and so on. So you could get something like three hundred pounds for one day of work, which sounds pretty good, but don't forget that, in commercials, they also have to pay a buyout on top of whatever they pay for the day (or days) of the shooting, to cover image rights. Sometimes it is a six month buyout, it could also be a one year buyout - if they want to continue using the advert after that period, then they will have to pay you again.
In theatre, for the same role, you can get different ammounts, depending on many factors, such as the location of the venue - you are likely to get more money if it's a West End production.
Basically, a bit of research online should give you a pretty good idea of what is fair and what's not. And talk to your friends in the business, ask them how much they are getting.