Mainly its signs of commitment. It's very easy for some guys just to say "I love you" and not really mean it. Look for actions not words.
Anything where they're sacrificing money that they could have used to get you something meaningful, spending time with you, being there to comfort and reassure you and prioritising your needs above others is all some signs of love.
They should also be expressed over a long period of time and consistently, not one or two occasions.