Who should be the first one to slip the tongue???

Ok, my girlfriend and i have been going out for like 2 and a half months and . we kiss and all but i am ready for Frenching with her and its not MY first time frenching but i think it might be hers. My question is, who should be the first to slip in the tongue? she is the first girl i have had any serious feelings for, and i think that this is why i don't want to do anything that may even possible do anything to jepordise what i have with her. the reason i haven't already tried is because i don't want to do anything shes not ready for, and push her away from me or make her feel uncomfortable around me. if anyone knows of any signs i should look for or what i would apprecieate it. Thanks-Mitch.