You should be able to tell what the sockets are visually. If in doubt take one off and go visit your nearest large caravan parts dealer. They may have matching parts.
Ideally you want a diagram of your electrics, perhaps bailey can supply one, or you can map your own. Ideally you need to know is how many sockets are on each spur and the wire diameter. If for example you have 2 x 2 sockets spurs and a 1 socket spur all on the same flex then it's fairly safe to assume you can go with another socket on the 1. Adding to the twos in this scenario means considering the wire load capacity and what you will run on it. The actual process for changing sockets is relatively straightforward hole drilling and rewiring as long as access is available.