None that can be trusted as sure. Men (and women too) are simply too complex to have such a simplistic definition system. The only way you will find out if he's bisexual, gay or straight is to ask him.
There may not be any specific signs, some people are more open to these things than others are.
Additional answer: there is no reason to be "alerted" to a woman's bisexual orientation than to be "alerted" to her heterosexuality or homosexuality. You use the term "alert" as if something is wrong with the woman's sexual orientation and it needs some kind of public warning made. There are 3 main categories of sexual orientation for humanity and the animal kingdom alike: homosexual, heterosexual and bisexual. There may be different numbers of gay, straight and bi people in the world, but they are all three normal. Nothing to be warned or on the alert about.
You are attracted sexually/romantically with both genders, you can sway a little more to one side and still be bisexual. Also you find people of your own gender and those out of it attractive.
Basically if you are attracted to the opposite sex AND the same sex as you.
Journal of Bisexuality was created in 2000.
Since lesbians don't believe that bisexuality exist,only men and women with bisexual tendencies should answer questions about bisexuality.
Since heterosexual men believe that bisexuality does not exist, only men and women with bisexual tendencies should answer questions about bisexuality.
Since heterosexual women don't believe that bisexuality exist, only men and women with bisexual tendencies should answer questions about bisexuality.
Since lesbians believe that female bisexuality does not exist, only women that have bisexual tendencies and only bisexual women should be allowed to answer questions about female bisexuality.
Since gay men don't believe that bisexuality exist,only men that have bisexual tendencies and only bisexual men should be allowed to answer questions about male bisexuality.
Bisexuality
It is legal to engage in heterosexual or homosexual relations in Greece, but the phenomenon of bisexuality is still very misunderstood and condemned by many Greeks.
Bisexuality does exist. Gay men and women do not have some kind of authority that allows them to say otherwise.
Being bi-curious means that you are straight, but are interested in/curious about being with members of the same sex. You are basically curious about both sexes.
There is no connection between bisexuality and mathematics.
No. There is No bisexuality in the SEGA Sonic the hedgehog games.