When she says, “I should have married <insert random name of one of your buddies>”, the relationship isn’t going well.
When she says, "<Insert random name of one of your buddies> has a bigger d–k than you, the relationship isn’t going well…
When she clubs you in the back of the head with a Thermos jug as you are going downstairs, the relationship isn’t going well.
When your son looks exactly like your best friend, the relationship is definitely on the rocks.
Does it have to be a Thermos jug, or can any blunt object be used?
Some less tongue in cheek ones, based on personal experience:
If you’re going out with him because you feel sorry for him because he likes you so very much but you don’t reciprocate his feelings, it’s time to reevaluate the relationship.
If you dread getting phone calls from him and wish he would be the first to hang up, that’s not a good sign.
If snuggling with him creeps you out more than it turns you on, trust your instincts and leave the room.
EDIT One more:
If he calls you afterwards, wanting to know what went wrong with the relationship despite you spending an hour and a half explaining the above three points, get a restraining order.
My direct experience was with a Thermos jug; I suppose I’m lucky she didn’t have an axe handy. Anyway, almost any object, blunt or sharp, would have done at the moment.