You can get a general idea of someones attitude, knowledge, decision making, and risk tolerance just by talking to them. Questions you should be asking yourself are
Does he sketch me out?
Has he been here before?
Has he done this before?
Can he explain why he does something a certain way, or is he just parroting?
Do I trust this person's decisions?
Can this person manage the situation/get help if I get injured?
This gives you a sense of what to expect from a particular partner. Talking in person is best. Talking on the phone gives you less information, and talking on the internet gives you even less information.
The above gives you an idea of whether or not to get out with a person. Let's assume your comfortable with this guy, have a little confidence in him and yourself, and have decided to go out on a trip, climb, cragging, what ever it may be.
Now at the climb/crag/route, you continue observing and talking to your new partner. If something doesn't look right, ask. It doesn't matter how long you've been climbing with another person, or how much more experienced he is- speak up. Are you worried about rock fall? AVI conditions? Difficulty of a route? Anything related to safety? Speak up. Talk it over with your partner. If you disagree, explain your reasoning. If you can't agree, the most conservative, risk averse decision should be the default.
If you're in a new area/environment (you said you were new to vertical rock) Then you are leaning on your partners experience and judgement. New to anchors? Read up on them in Freedom of the Hills or watch some instructional videos on youtube. Cam placement? Same. Giving yourself a basic understanding allows you to have discussions with your partner and will better arm you to spot red flags.
When selecting a partner, the three most important things to me are my partners judgement, compatible risk tolerance, and if our combined experience is adequate for the objective.
I've climbed with several strangers in the gym, at the crag, and on big mountains. The process is the same.
With any new partner, I have them demonstrate to me how they belay. It maybe awkward, some might be insulted, but if my life is in their hands, I like to know how they'll use them.
