How to know if you have severe dehydration?

No judgment cause I’m already surprised that I forgot to drink water, except for ice tea and half a small bottle of water in the early morning where I live is really hot, and I went to work at 8 and returned at 4 that’s when I noticed my mouth was dry asf. I panicked and immediately drank water because my friend passed out from dehydration before and didn’t look pleasant at all.

My mouth was still dry and I drank pineapple juice (with some salt) and like 400 ml (or more) of water. After that my pee was clear and the dry mouth went away. I took a 5 hour nap and I woke up with a fast heart rate and a dry mouth and a weak feeling I went to pee and it was little pee. How do I know that it isn’t an emergency? I also drank soup and orange juice to ensure I’m okay.