How to tell if a cat is about to pass?

Okay, so we have this cat, and her name is Carla. Carla is older than me, being 13 or 14 years old. Making her age in cat years 78 or 84. I don't think anyone else realizes this, but she's getting very fragile. Our cat Woody was 11 or 12 when he died, so I think Carla may be starting to go too. When I just sit and listen to her breath, it sounds forced and in shallow breaths. She eats and drinks fine, and she uses the bathroom just fine. But the thing is, sometimes while she's sleeping, she looks like she stopped breathing, and I'll get worried that she's dying, so I shake her and everything and sometimes she takes a loooooong time to wake up, and that scares me a lot. So I want to be prepared for when she's actually passing. Do you know of any signs that can tell me when I actually need to be worried?