I've always just told people when I feel that early "puppy love", which usually is a bit of a shock to them. I hate that term because it implies it's not real love, but I do recognize that it's different from the feeling of love that comes later, when you're more clear headed about the future of your relationship.

As to fears about scaring them off by saying it to early, it's a reasonable fear. People can be scared of commitment, and if they feel like things are moving too fast they could pull away. I think the right way to handle it is to feel them out by telling them romantic things that aren't a blunt, "I love you", and see how they react. Move it forward at a pace they seem comfortable with.