So my mum bought a LEGO set on eBay a while ago and it didn't come with instructions or a box so we didn't know how to build it but since then it has been jumbled up with a whole bunch of other pieces so I don't know what's in the set and what isn't. Also, I think it's the from Belville range (https://en.wikipedia.org/wiki/Belville_(Lego)).
Start by looking for some rarer pieces. Not regular 4x2 bricks, but maybe a slide or a odd-shaped wall piece. Under each brick will be a four or 5 digit number. Go to BrickSet.com and search for that number, hopefully it will find the piece you are looking at and list colours that piece is available in and the sets it comes in (e.g. http://brickset.com/parts/design-6187). From there you just look up and down the list until you see a set that contains the other pieces you have, or search for another part and see if there are sets from the first list that appear on the second list. The trick is to find a part that is sufficiently rare.