There are couple women I'm interested in a couple go sleep and would like to find their sexuality with out asking. The only hint I have is fb One has interested in checked to men and the other has hers blank. The reason I ask there has been times when assumed someone was straight because they had their profile checked to interested in to opposite gender and I later found they were gay/bi
There's no way to reliably tell without asking, unless the person happens to wear a rainbow bracelet or something.
The one that keeps it blank is your best bet I think. I kept my fb blank when I had not officially come out because I did not want to call myself straight but did not have experience with women so I felt awkward calling myself anything else.