To know you're in one you check your gauges regularly regarding depth. Or notice the change bylooking at a point of reference. I have a little pet peeve with divers who don't look at their gauges often enough (wrote about it in fact in The Scuba Snobs Guide to Diving Etiquette). Part of being a good diver and a safe diver is being aware of where you are, how long you have been there, and if you need to move- especially to shallower venues. The comments above are all excellent, to which I will add a few obvious ones- step one, kick up to get out of the current; step 2, if kicking alone isn't making it, inflate your bcd to get some lift, but control your ascent rate as you were taught back on day one of your training; as you move, move across and away from the current- as noted above, that usually means away from the wall if there is one; and, step 3, in an extreme case where the current is so strong that an inflated BC and kicking like hell doesn't work, drop weight. Perhaps most important of all, be aware of conditions before beginning a dive through checking with the locals and as appropriate the coast guard or equivalent authorities, as prevention and avoidance is best. Finally, take evasive action at the first indication of a down current, however slight, if you are in "bottomless" water.
DivemasterDennis scubasnobs.com