Ignoring the outer fringes of the rarity of a social observer, or just seeking a hook-up, most are there to dance (there are other reasons). Yes, there may be selective choice of dance partner of those actively participating in the specific game, and here we're back to the request/response of cabeceo/mirada, but, as discussed at length in other threads, there's then the non-maskable verbal or walkaceo request. Sure, there are restrictive cohorts, and in some locales it's not socially acceptable (codigos) to request by any means from outside the cohort, but it's most likely about a milonga in a large, western, city - presumably 'central' location for tourists and probably with a younger clientele. Even a basic social politeness response to a stranger should get at least one tanda, or maybe at worst an outright rejection. But then, at least, you know. As a visitor, does that even matter? You're unlikely to return.