[rocket_surgeron]

If your primary care provider won't do at least one vitamin-d test per year, and then periodic tests once a deficiency is identified, you should get a new primary care provider.

If that's not an option, non-prescription at-home tests are available in the US for as low as $49.

I don't know about monthly, I think a 60-day followup after treatment in order to titrate your supplementation might be best (it takes time for supplementation to work), but at-home tests are an option if your provider is for some inexplicable reason, reluctant.

As far as I can tell, the only harm from supplementation comes when the typical adult consistently consumes doses so high as to be absurd (50,000+ IU daily for months) so I don't know why any medical professional would be hesitant to investigate such a common health issue with such an easy, inexpensive, and effective treatment.