It's usually a simple matter of checking the tooltips on new actions, but not all jobs with positional requirements have their positional requirements from the moment they get a relevant weaponskill all the way until 70. In the case of SAM, Gekko and Kasha don't have positional requirements until 52 when they get the trait Kenki Mastery, whose description is nonspecific: "Increases Kenki Gauge when landing select weaponskills." At this point, a SAM would need to check every single weaponskill if he wanted to determine which "select weaponskills" were affected, and even that would first require that they care enough to check. But since Kenki Mastery doesn't say anything about positionals, a new SAM might assume that kenki was simply added to certain weaponskills without their mechanics changing, and to be honest, that would be a fair assumption.
Other jobs where positional skills always have positionals from the start, there's not really an excuse for making it to Lv70 without knowing that weaponskills have positionals.