You've been working out with hand weights for a while now, and no doubt you had perfect form when you first learned your favorite exercises.
Ahhh. . . but repetition is a slippery slope and often when we repeat exercises, we slowly stop paying attention to form. Over time, bad habits have a way of working themselves into your routine. This being said, it is a good idea to check your form every once in a while. There's no need to watch yourself every time you do your bicep curls or side arm raises, but once every few week or so, watch yourself in a mirror to make sure you're keeping up with that perfect form.