hahahahahahahahaha, WOW. theres been like 2 people here who understand how this all works.
ALL SKIS come from the factory with a factory wax. this factory wax is nothing more than rolled on parafin, which is what ski wax is, minus all the chemicals and additives that help with speed. it is there solely to prevent the skis from shipping with a dry base. it doesnt last, and is not at all fast.
and yes. when your skis are new, you should hotwax them. ALOT. the first few waxes are super importnant to building up the base, and waxing them multiple times really helps contaminate the base with wax. so far, i have waxed my skis EVERY time i have ridden this season.
now, as for how to tell when your skis need waxed. when your base becomes dry, it needs wax. dry bases look like there is a white grain over the base, versus glossy. they also feel dry to the touch. a waxed ski is pretty much frictionless feeling, and is shinny.
and yes, waxing a ski makes a HUGE difference in the preformance. especially on slower hills. there, the added glide is very noticable. so bottom line, you really should wax your skis. and if you cant do it yourself, learn. its incredibly easy. and it beats spending 30 + for a shop to do it.