I have genital HSV-1 and I got it from an ex-boyfriend who got cold sores. At the time we didn't know you could transmit HSV-1 orally-genitally, so it wasn't even on the radar.
He performed oral sex a number of times (never during an outbreak or when anything was visible), and all was fine.
A few months after we broke up we hooked up again, and bam... a week later I was in the midst of a terrible initial outbreak. I was diagnosed w/ herpes from the culture test, but when they did my bloodwork it came back negative for both (meaning it was a new infection). When I got my bloodwork done again 3 months later (they say it usually takes 12-16 weeks for the antibodies to show up in a blood test) I was positive for HSV-1.
It's important to note that even though we didn't know there was a risk, he had not had an outbreak recently, did not have any of the symptoms like tingling around that time, and didn't break out for months after either.
The problem is asymptomatic shedding... a very small percentage of the time the virus is active and transmittable even if there are no outward signs or symptoms.
So bottom line is, you can never know. It turns out we were gambling for months, and were lucky; now I have something that I have to live with for the rest of my life.
What are your options? I urge you and your girlfriend to read as much as possible. Personally my infection hasn't been too bad--physically. HSV-1 is much milder when it's contracted genitally, and I've never had an outbreak after my initial one (though that one was a doozy). However, it's very difficult emotionally and socially.
If she hasn't contracted it yet, you can try to use dental dams during oral sex, and read more on that kind of stuff. Also, there are supposedly lubricants that are supposed to help prevent transmission--http://www.natropractica.com/ Christopher sells one that he has developed. I don't know if this appeals to you, but I've purchased some (haven't used it yet).