After 30 years, I can pretty well read him. Like a book.
But there are other ways to tell..
1. If he ever stopped going 'ooh ooh' over girls on T.V. That would be an indication.
2. If he were suddenly to take an interest in other men - more than just tools and stuff.. Or spending more time with 'men friends' than with me.. Then I'd start to wonder
But it takes more than just one sign to really know for sure
Happier and more flirtatious around handsome straight men than he is around you and other women
The sexier, more masculine/aggressive-looking the straight men around him the happier and giddier he seems
More than 25 percent of his Facebook friends gay males
Avoids PIV sex with you and other women
Absolutely repulsed by performing oral sex on you or other women and outright refuses to do so, under any circumstances, ever
Skewed perception of women's bodies (e.g. Kate Upton = "fat" or "needs to lose some pounds")
A certain instinct radar going off in you that won't shut up or go away