How can I clean white couches
I like Folex for cleaning upholstery. but I would spot test it first to make sure it doesn't leave marks.
What fabric are they covered with?
Depends on what they are made of. If timber - Soapy water and a nail brush. If Plastic - you could use UPVC cleaner ( used to clean window frames etc.
For upholstery I use a handheld Bissell cleaner. It can be used for spot cleaning on carpets as well but works great with upholstery. You will have to go over the whole seat though because you don't want water marks which can happen with white fabric. The great thing is that after you spray clean with the cleaner and lightly scrub wherever there are spots, everything gets sucked back up. Good luck!
My husband rents a carpet shampooer from our local grocery store for a couple of days. Then he uses the attachment to clean our white microfiber couches with some solution and he uses Spot Shock (I believe that is what it is called) for extra problem areas. Works like a charm!