How can I tell which fruits etc. contain proteases?
Enzymes which degrade proteins, called proteases, are found in many fruits. There is no simple test for it other than holding some in your mouth and seeing if it "eats" your flesh away after a few minutes

Commercial test are not practical or portable as they require maceration, heating, centrifuging and using florescent dye markers. Or just make some in gelatine and see what happens :-)

A quick list would be:


Fig - Ficin
Ginger - zingipain
Kiwi fruit - Actinidin
Papaya (Pawpaw) - Papain
Pineapple - Bromelain


Also to a lesser extent


Banana
Guava 
Mango


It is in many other fruits, but at levels where they will not be a problem, of course at different levels depending on the specific fruit variety etc.