DISCLAIMER: I ALWAYS recommend trying things on. Especially when shopping for vintage. Things are cut a lot differently than modern day clothes and 99% of vintage/thrift/resale stores are final sale. That being said, here are a few things to think about if you can't try it on.
1. Know your measurements.
Lots of vintage stores don't write sizes on tags because the sizing system back in the day is so much different today. Zero, two, and four didn't exist until not that long ago. The measurements you should know are your bust, waist, and hip. For bust, measure around your body at your boobs fullest point. For waist, find the natural indentation a couple inches above your hip bone. For hips, find your leg joint (should be your widest part).
2. If it can wrap around your neck, it can fit your waist.
This sounds weird but it's true. When looking for pants, wrap the waist band around your neck. If it touches in the back, you're golden.
3. The distance from the side of your neck to the tips of your fingers is the same as your inseam.
The inseam of pants is the measurement of the crotch to the hem on the ankle. This is a good one if you're looking at pants but you're worried about the length.