How can you tell if you are annoying the girl?

Well other than outright telling you, it'd be how they act around you.
If they don't seem interested in what you say or just give you a look when you walk by, that would be a sign.
When I'm annoyed at a guy I just ignore what they say and somehow shun them off (yeah I'm not that nice) but I guess sometimes it's not as obvious as it seems.

She might make weird faces and say LEAVE ME ALONE!!! That's an obvious sign but she will make weird faces and just give YOU weird mean looks and ask you to stop.

If you're annoying a girl she might act sarcastic... in a mean way... and not really care what you have to say. She'll change the subject or just not put any effort into your conversation... she might look away or look at the time a lot because she wants to be somewhere else. If you feel like you're annoying a girl or that she's just not interested, than go with your gut feeling on that, because it just might be so.

If I'm not interested I tend to change the subject, but if I'm annoyed, I get a little quiet and seem to get nervous. But, I think it depends all on the girl. If they are outspoken I think they would let you know. If you are with a shy girl, like me..I just get quiet and start to not know what to say, and usually most the time I want to leave or get away as quick as possible, or just not talk about it anymore.

She'll tell you... trust me. This one time, in band camp, this chick told me to f**k off. I cracked up laughing but then left cos I got the message =D

Practice reading body language and facial expressions. Try this exercise: When you look at people (from afar, up close, this even works looking at people on tv) try to mute what they are saying and only read what their body language and facial expressions are saying and communicating. ex: "I'm happy. I'm sad. I'm confused. Annoyed. I liked what you said, I didn't etc."

A lot of times people will say one thing and react another.

Emotional *reactions* are very close to mind reading. It takes time to think up body language to deceive people, reactions are instant and very often the truth.

When you interact with people read what their body, face, eyes, voice tone, and words are saying. There are plenty body language and interpersonal relationship books on this. A lot of this is why women are seen as very intuitive, they pay attention to all the details.

But least you need to know: study people's reactions,(including your own), how you effect people emotionally and how they effect you emotionally. And when in doubt, just ask them.