my dogs trying to mate

Question
and how exactly do u check that ? like what am i suppose to feel for? or it that what you mean?
-------------------------------------------
The text above is a follow-up to ...

-----Question-----
so my female boxer has been in heat and i think she is ready to mate now as she is standing there waiting for my to flirting with him shes 'flagging' him but she lets him get on her and everything but when i think there fixing to do the thing' he gets going then my female ends up welping like he hurt her and jerks away ...any advice on why shes does this? and this is her first time trying to mate both of them are about 2 years of age. is there anything that i should be concerned about?thanks so much any advice is helpfull
-----Answer-----
Hi Brittany,

Did you have an internal done before breeding?  Her hymen could be intact or she could have a constricting ring that could prevent normal entry. A maiden should be examined prior to breeding to ensure her vaginal orifice is a normal size.

You can check yourself by insert your clean finger.

Answer
Brittany ---

You just want to feel how tight it is. If it feels tight on your finger or you reach a restriction, there may be a problem. You can have the vet check it if you are uncomfortable.

Some females will yelp the first time and may require assistance by holding the female.