Different in every relationship I've been and assumed that what I feel is "love."
For one thing, love feels different than the feeling of being in love.
Love can be unconditional - painful. Ouch. Bad. Bleh.
Love can be that p-word which escapes my mind at the moment.
And it can be mutual.
The feeling of love is caring for someone, having compassion for them and being empathetic.
Being in love; the deal breaker for me, knowing that I'm in love with someone, is when I think of death and to calm myself down I think of them; the thought of being without them makes my arms ache as if I had a panic attack coming on (sounds really clingy but it's just something that happens to me xD); and ultimately the proper proper way I can make sure I'm in love with someone is knowing and having the feeling that they are real. Like, realizing that they are real, they're alive, they actually exist. You know that feeling where you're like "what if everyone else is fake... and it's just me?" yeah, well, with people you love/are in love with, that goes away if it's proper. At least, that's how I see it.
It's a feeling of contentedness with life; I hesitate to say "perpetual happiness" because you can still be unhappy with certain aspects of your life while you love/are in love with someone. But always having a background feeling of being content, a feeling of being safe, loved back, all round warm inside, I guess - the usual stuff people say about it. Along with feelings of wanting them and needing them.
Also a feeling of selflessness has to accompany it. If I will do just about anything for that person without questioning it or complaining (unless I'm joking ;D) and I'll just get stuff for them without having to ask why then yeah that's another part of it for me.
Long answer is long.