Do a blood test. All vegetarians should do blood tests every now and then.

As an aside; I ate a vegan diet for 2-3 years and my B12 was fine. That’s not to say it would’ve been fine 2-3 years later, though.

Like the other commenter said— it was a blood test. The result was actually low hemoglobin. Low hemo unfortunately runs in the family as well– my mother has a severe case of anemia and suffers a range of issues related to that. Thankfully, I'm nowhere near that case.