If you're waiting for a sign, this is it. Go tell them that you love them, or do whatever you need to do to get them to realize just how serious you are about your feelings. (Well, not anything. Anything they're comfortable with!) Just do it. This is your shot. This is your chance. Just do it. This might be the only shot you've got. And if things don't work out, then you don't need to spend a second more of your precious time on them, because it's not the right time and it wasn't mean to be now. There might be someone else out there, or you might need to spend your time on YOU for a while instead. So go and do it--before you lose the opportunity to someone who reaches out boldly with both hands. You deserve to go for it. So do it!
This is your sign.