With your hand. reach and squeese Well first depending on how your gf is first i would play around with her and just slitly hit her butt and see what she says or looks if she smiles or laughs that's...
no, unless you have been dating for atleast 5-7 months and have a good relationship(must be girlfriend or it will be weird) try doing it when kissing your girlfriend No there is no limit on how long...