It isn't real simple anymore, but, not hard either
Modern foam surrounds have built in UV resistance that the old ones didn't. The ones used in most speakers of any quality whatsoever will last 25, 30 years, or more. Butyls will last for 40 or 50. Don't see much of the old type foam anymore, although I did see it on one really cheap KLH at Best Buy the other day. The speaker someone was going on about being "really good for a really cheap speaker". Like $50/pr bookshelves. They still had a foam that was discoloring, meaning it wasn't very environmentally resistant. There are also some compressed foam surrounds that look like butyl rubber but really aren't.
Best advice is not to worry about it too much. Buy a good set of speakers and enjoy life.