1. When all her friends are guys.
2. When she does hard drugs or even smokes alot.
3. When she gets drunk alot and doesn't remember much the following morning.
4. When she sees a therapist or on some sort of medication.
5. When she is overweight or too skinny.
6. When she has outbursts of anger or she cries alot. Actually any emotion that she shows too much of then she got issues (I once knew a girl that smiled and laughed on everything I said, that was the creepiest thing ever).
7. When she lies alot.
8. When she doesn't want to fvck you at all or she wants to fvck all the time (the latter is not really an issue for the guy

but it could be if you aren't around then she might be looking for d1ck somewhere else).
9. When she complains and nags all the time.
10. When she wants to be in the centre of attention all the time.
11. When she wants to have the last word all the time.