I need help

😀 [i](I know what you’re thinking)[/i]

Seriously though, how do you know if a cell phone can be legitimately used with a service if you purchase it through a 3rd party?

My SO does not want to be locked into another 1 or 2 year contract but we need new phones. Therefore I’m taking a peek online, but before I buy I want to know how can I be absolutely sure it can be used with our service? I mean, other than going by the seller’s word.