There are always *signs* that you are dealing with someone who isn't sure of themselves, don't know who they are sexually or WHAT they want. Like a lot of things, people see what they 
want to see, while ignoring red flags.
 For example: gay guy starts casually "seeing" a "straight" guy who 
thinks he "may" be bi...but "still wants to be in a relationship with and ultimately marry a woman someday...
 *buzzer sound*
 GET OUT NOW.
 *"bisexual" guy who fucks guys, but isn't ready to "settle down" with a dude yet. 
 *"bisexual" guy who is sexually attracted to guys, but emotionally attracted to women.
 *"bisexual" guy who is sexually attracted to guys, but emotionally attracted to women. 
 *a previously-straight guy whom you "turned" gay. 

 
 *a bisexual guy who is fine with being intimate, emotional, etc. with you...but won't do anything remotely "open" (talking to you about his life, family, introducing you to friends, etc.) that also goes for GAY guys (closeted gays, that is...) as well...
 You get the picture. That's only a few scenarios. There are many more with different variables across the board.
Be discerning and PAY attention to people's 
actions.
 While some guys are great at playing a game and pretending to be someone they are not, they will almost always show their true colors with time--and usually not long after meeting them. It's not always immediate, but kick rocks as soon as you start seeing the bullshit. It's up to the individuals involved to decide how long they want to stay along for the ride. 
I give newbies to the gay scene a little leeway, because it usually takes some time before one can easily discern such fuckery. 
There are no concrete rules, but there will be red flags that shouldn't go unchecked. 
When someone knows who they are, and are comfortable with who they are and confident in what they want in a mate (such as TRUE bisexuals)...you know. Their actions will show it--almost in the complete opposite manifestations (on a constant basis) of the examples I mentioned above.