This is two separate issues. The first is being aware of how much you can actually manage. The second is having the capability to refuse requests.
For Issue 2, here's a rule of thumb:
If you find yourself wishing they hadn't asked you in the first place, you should probably say no.
That way you'll be able to distinguish situations from when you actually think you should assist vs. when you feel obligated. (If you actually think you need to assist, you should. Don't go the opposite direction of always saying no.) Also, this advice assumes you aren't playing any signalling games.
I'd say it is a matter of noticing apprehension at the prospect of saying yes, and listening to it.
Some people seem to be a bit too generous for their own good. I know a precious few people who are especially good at saying "no" when asked to take on new responsibilities that would put them over their limits. I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing. I expect this to be an extremely valuable skill it would probably be good for many of us to understand, learn, and be able to teach to people who really need it.