When do you know and other than losing interest, that it's a no go for a relationship?

I don't know because I have never had my only serious relationship endangered. Before we married, she cheated, lied, and probably did other things, but I didn't love her any less or think about breaking up with her.

Generally if I have to beg or struggle for attention I will get the message and that feeling of being undesired is enough to kill my own desire in return.

When they are nonchalant / distant

when you can't stand to be in the same room with them

It's when you start shutting down .

Dishonesty, disrespect, arrogance, rudeness, game playing, controlling, humorless, negativity.