how ne can identitfy g-spot ...ven he is in intercouirse or insert fingers into vagina ?

i used to hav to sex with my girl frnd,,,,,my session is completing early in first time but se is not ...in 2nd time ...she is completing first and next me ....i used to insert fingers into vagina,,,,,how can knw i reached g-spot ?
one more thing ...
ven we had sex 1st time ...my penis was gone fully inside and she said that im the the first person ,,,,,,,is it true?
if im the first one ..it should not go fully deep..............is she virgin ?.