How can i find out a person's blood genotype? If the person has a B+ phenotype, how do i know if the genotype

You can't know that without a genotyping assay.  Or crossing the individual with a known genotype and looking at the phenotypes of the offspring.  Remember that the O notation means no surface antigen on one locus, and the B notation means a B antigen on the other locus.  B/O perfectly phenocopies B/B.  A quantitative PCR of the B-blood type gene could be used as a genotyping...because the PCR band for a B/B individual would be twice as bright as a B/O individual...though you would need a known control for each.  Sorry, I hope this helps!

I agree... You can't know without more information.

The best thing would be to cross with an |^O|^O.

If all offspring have phenotype B, then you know the cross was:
|^B|^B x |^O|^O = four of |^B|^O

If half the offspring are phenotype O, then you had:
|^B|^O x |^O|^O = two of |^B|^O, and two of |^O|^O