I'm starting to plan towards an eventual castration and plan on going on HRT. I think it might be wise to figure out what my normal T level is before I start trying to kill my balls. My concern is that if I go to my doctor and ask them to check my T level and come back a year or several months later and it's drastically lower they would suspect the change was intentional. How do you recommend finding what's normal for me, and determining if any attempts to kill my balls are working.

You can always changes Doctors.

But don't think it really matters what it is today.

They will prescribe a normal dose and check levels in blood and make adjustments as needed. See how you feel and try to maintain in normal range

You don't need doctor to check test levels. There are lots of online analysis services. You just pay online and then go to a local blood draw clinic and they will give you your results directly. Not too expensive either.