How do you know when you have intense feelings for someone whether it is love? Is that your question?
Welcome to the collective, by the way.
A number of factors come into play.
One is time. If you have just met someone you may be strongly attracted to them, you may be infatuated, you may be in lust, but that is not love.
The initial chemical haze of attraction (because that’s what it is) fades in all cases. Love is something that grows over time as you experience things with someone, share things, learn about each other.
If you truly love someone, their happiness matters more to you, or at least as much to you, as your own. You want them to be happy, to succeed in life and their endeavors. You want to build a life with them, share goals, handle adversity.