I guess one thing to look out for is the smell of rotten eggs in a server room. From what i understand that is a sign that the UPS batteries are venting hydrogen sulfide.
That or sulfur dioxide, yeah. I've smelled the latter in an office that doubled as a data center, when one of the UPSs' charge regulation circuit went wonky and overdrove its batteries.
That's probably not a major asphyxiation risk unless you're trapped in a room with it, though, because, unlike inert gases like helium or nitrogen, sulfur compounds are irritating as hell - a UPS boiling its batteries will send you fleeing for the nearest source of fresh air, with eyes and nose streaming, before it'll suffocate you.
