Hi, silver reeds, welcome to Psych Central!

As far as I can tell, there are no signs; or rather, what any given "sign" means, is going to depend a lot on the context. If you find yourself missing someone, it could be that they're drifting away from you, or that you've drifted away from them and you don't like it, or that you're worrying about what would happen if you and they did drift apart... etc.

Less phone calling could mean that one or the other of you is losing interest -- and/or that one or the other of you is busier than usual with other things, and/or that you're feeling more secure with each other and don't need to reassure each other as often that you're still there.

I'd recommend that you (1.) look and see what any such "signs" seem to be saying to you; (2.) don't believe everything you think, especially not the first time you happen to think it; (3.) look for other indications, in your own experience as well as around you, that the "signs" do or don't fit with what else you know; and (4.) once you have a pretty good idea just what it is that you want to know, try asking the other person.

For me, it's much more common for any two people to cross paths briefly, then resume going in their respective directions, than it is for them to find themselves following the same path for long. When the latter does happen, I'd call it a sign that both of them chose it that way.