The easy answer is to check who's in tank stance vs dps stance. If no one is in tank stance either step up and be the MT, or if you're new/don't know the content, maybe ask one of the other tanks if they wouldn't mind MT'ing. When I run stuff myself and the other tank(s) aren't in tank stance, I usually ask "(name), do you prefer to MT or OT?" and that either gets a "I'd prefer to X" or a "which would you prefer?" response