There is a blood test. But the interpretation of it is questionable.

It will reveal very low levels, and that is more accurate than the meaning of high numbers. Anyone taking a supplement will show elevated levels. The lab ranges were determined statistically on people who are NOT taking the B6 extra. So higher levels don't necessarily mean "toxic", but only reflect on the extra intake.

I did find an interesting paper on autistic children, who were found to have elevated B6 in the ABSENCE of supplements. The reason for this is not understood at this time.
The safe upper limit for B6 is 100mg a day at this time. But people vary and we have seen some here at 50mg a day or a bit less who have side effects.

B6 metabolism is genetically mediated. In some cases of babies born with seizures, called infantile spasms, they are given really high doses to maintain a seizure free life. In this case it is called vitamin DEPENDENCY. Vitamin dependency can occur with anything given in really high doses. For example really high Vit C (which was a fad of sorts when Linus Pauling was still alive) resulted in scurvy symptoms with suddenly stopped. It is now suggested that high dose anything should be tappered when it is decided to stop it.