You cannot see your cervix with a mirror, it is too far up your vagina.
If you have ever had a Papmear you will remember the doctor/nurse doing it would have used a speculum to find your cervix.
I urge women to gentle check out their own anatomy. See if you can find your cervix.
To do a VE and examine dilatation during labour requires a 2 fingers to be put up into your vagina.

Why would you need to check in early labour....would it change your plan? I mean would you go into hospital sooner or stay home longer based on what you find...and how will you know if you are right? The first few VE's i ever did as a student midwife felt just like mush to me, I had no idea for a while what I was meant to be finding.
It makes more sense to base the progress of normal labour on strength, regularity of conractions. Trust your instincts.