Your emotions about someone are and can only be controlled by YOU. Someone else cannot tell you how you feel about someone. But just a common "theory" most people use is that if you have feelings for someone, you would always want to be with them and if it's the other way around, you wouldn't want any part of them. But no one can tell you what your emotions for someone are. Only you know. Goodie luck :)
You don't need to know from any external source. It will be told by your heart beat
If you posted that question your question not only answers itself but says to you that that person means something to you.
Well if you still have feelings for them, you keep thinking about them and wondering what they are doing but if you don't anymore, you wouldn't care if you saw them walk past you lol.