Petite mutants usually lack functioning mitochondria and grow more slowly, so the colonies they make are smaller than normal colonies. They are petite.
Plating is good for looking at yeast health, even if you don't use the yeast from the plate. There are a couple of ways you can do it. The main thing is that you want to isolate single cells to examine the colonies after they grow big enough. You want the colonies to be smooth, round, and uniform in color. Two different strains may have different growth patterns, but you'll become familiar with them.
The easiest thing to do might be to streak for singles. You put a small dot of yeast on the plate and smear it around. I could go into detail but MB has it covered here, under "Agar Plates"http://www.maltosefalcons.com/tech/yeast-propagation-and-maintenance-principles-and-practices
The other option is to dilute it so the liquid you add to the plate doesn't contain many cells. It can be hard to do at home because it is difficult to spread the liquid around without a sterile spreader and you would tend to add too much liquid so the yeast starts growing while it is evaporating, which causes smeared colonies.
Anyway, I'm trying to figure out if 1450 has a tendency to become petite or if I just have bad luck.