There's always craigslist, or any of the other local online sites in your area.
The reality is that you're asking, in a way, to have your cake and eat it too. You can't hook-up risk-free; and you always run the risk of being 'outed' even if the other guy claims to be discreet. Generally, most 'out of the closet' gays and bi's would, given the choice, prefer some sort of a relationship where they don't have to lie about their lives (and I'm not making a value judgment on that, it's just a reality that if you're closeted, chances are you're lying to someone about some aspect of your life) because that's what coming out is ultimately an expression of--the unwillingness to lie about one's sexuality.
This obviously isn't an absolute rule and there are definitely openly gay guys who don't mind hooking up with married guys (whether they identify as straight, gay, or bi), or closeted guys; and there are even ones who don't mind keeping it a secret that you've slept together; or that you're attracted to other men enough to have sex with them. But if it's that important to be closeted, then likely, your risk is mitigated if the other guy is also closeted, since you both will have an incentive to keep the other guy's secret.
But to answer your question most directly, there isn't a way to be SO subtle so as to keep yourself completely risk-free in your attempt to show a guy your interest. The operative word in, "showing interest" is "showing", which means you have to outwardly express something. Guys, even "discreet" ones, are not mind readers. Short of tapping your foot in the bathroom stall for a blowjob (where you could say you were trying to get something off your shoe), or going to your local pick-up park (where you can claim you were admiring the trees BEHIND that guy...), you must be unsubtle enough for the other party to receive a message.
The curious part about your question is that you also want the signal to be completely deniable. Why show interest to someone if you're just going to deny it if they show you (either by gesture or words) that they have not only received the message, but are reciprocating it? The only scenario that comes to mind is that you're not actually sure of his orientation, so you obviously want to avoid the "embarrassment" of being asked, "Um...were you hitting on me? Because I'm straight. Are you gay or something?" To which I can only say that nothing of value is wrought without risk--quite possibly especially when it comes to sex.