First impression of 30 year old virgin man?

How would I even know that he was a virgin and had never been in a relationship?

I can honestly say that I have never been informed about someone's sexual experience before learning what they do for a living or what hobbies they have.

You're defining too much of your identity with non/virginity.  Some guys just take their time getting started.  Try considering other characteristics first.

1) He's too cowardly to go as girls out.

2) He's probably ugly and girls don't want him.

3) He chose the Celibate life.

If I observe him and he's hitting on girls and they are turning him down, then I'll go with number 2. But I could usually tell from his looks and how passing girls looks at him if he's ugly to them or not.

If I observe him and he's not very talkative around girls, then he's probably too shy and too afraid to ask a girls out. Girls don't usually ask guys out even if they like him. They are waiting for him to ask them out and it wont happen if he's too afraid.

Number 3 is the hardest for me to identify. If he's hot and he turns away from flirtation from a girl, then he's most likely gay. But if he also shows interest in girls but chose not to pursue them, then that's probably because he chose the Celibate life. If he's ugly, then that makes it even harder because no one will be flirting with him for me to find out if he's taken the Celibate life.

You learn a lot from just observing people's behavior.