Female rats go into heat every 4 days, so there's a pretty good chance. Even if the "visit" was short, rats can mate in mere seconds.
You can weigh her daily, you can get her e-spayed, or you could, I guess, cross your fingers and hope for the best.
I don't recommend that!
Of course, if you do think she's pregnant and will allow the pregnancy to continue, I'd suggest making sure you have enough cages to separate the babies at5 weeks and start seeing if there's an interest in babies in your area.
Though you've been through this before? I guess you already know that.