The smaller plastic bag(s) of powder in these kits contains the developing agents. The powder should be nearly pure white. If it's yellow or brownish, then it has started to oxidise, the deeper brown the colour the more it has "gone off". A pale yellow is probably still OK to use. You might have to stretch the developing time a bit.(posted 9012 days ago)The chemicals in the larger bags shouldn't deteriorate to any great extent. The worst that can happen to these is that they lump together, and don't dissolve as readily as they should.
Make a batch up, and if the solution shows any colour at all, then don't risk it for anything precious.