Not "only", but it's not restricted by class, just whoever gets the training. Which given that it's standard training for any kind of soldier, means that it's very typical for commoners to be trained in it to act as soldiers and bodyguards and such, as nobles don't care to let their own kin dirty their hands with that sort of grunt work, after all. :)

....on the other hand, all magic-users in denyuuden world can literally see the magical energy they're manipulating. |D