Answer:
Tell them when you feel it! Well...when your sure you feel it and are not contemplating the feeling anymore. What may prepare you to tell them is using the word in other contexts. You can call them love as a bet name, for instance, " good morning love". See how they react to that. What works really good is if you SHOW them you love them before you tell them. This may help the other person say it back, because they can see that you care about them by what you do, and it won't come as a surprize. Also, remember that they may be contemplating about saying it to you first too! once again, you need to show them before saying it. Also, talking about your thoughts on love and relationships also helps you understand how they think of love, weather its casual or serious for them. I hope all this helps!!