Well the process of checking whether or not a woman is a virgin usually involves checking whether or not her hymen
is torn, it being assumed that the hymen can only be torn through intercourse.
The thing is that recent studies have shown that the hymen can be torn even by exercising or dancing, so its not a very reliable method.
As such I would recommend that asking your to be wife is the best way to go about things, as this test is considered to be very sexist and I believe might lead to more problems in your relationship than solving them.
I hope this helped.