Look at the electrolytic caps. They have a crease mark or marks on the exposed top of the aluminum can which is designed to break open and allow a controlled release of pressure rather than a pop when they go bad. Really bad ones will have opened enough to ooze goop either on top or around the bottom of the cap. Don't confuse the ooze with sealant sometimes used to anchor caps to the board. Ones going bad will often bulge the top. Tops need to be completely flat to be "probably" OK.
If you search on line for images of failed electrolytic caps you'll find lots of pictures of them. Here's an example of a good one and a bad one side by side. Sometimes the bad one will have oozed its orangish innards out.
https://us2.ixquick-proxy.com/do/spg...loatedcaps.jpg