Florenceuk, tofu is almost pure soya, so that's definitely off limits. Soya sauce is a bit of an unknown. It's important to avoid the protein and there's probably not a lot of it in soya sauce, but if you're doing an elimination diet, you should avoid it. Then reintroduce (after the initial test) and see what happens. Also, soya lecithin is ok for all but the most severe soya allergies. It's made from the oil (which is also ok, as long as you don't get the fancy cold filtered stuff.. if there is such a thing). My son could handle both of those. As I said, his allergy wasn't severe.
I had that discussion with my baby's allergist. It was in relation to my eating sushi, which I love, but need the soya sauce/ wasabi mixture to dip it in. She said it was ok, but don't overdo it.
Someone else who knows how soya sauce is made might be able to tell you if there's the protein in there.
Robinw, my guess is that they'll do skin tests (or blood tests, they're both about as accurate) and then go from there. If it comes up positive, then they'll either tell you to avoid it completely indefinitely or avoid it and then try it again after a few weeks. I think it depends on what the reaction on the skin is like. If you are testing for a lot of things, just know that there are many false positives, so don't be alarmed if something she eats every day comes up positive. Good luck! (And, just fyi, we're brushing with the xylitol toothpaste now and I've got the xylitol containing teething granules for my son-- thanks for the tips on that!)