There is an easy way to check if a skirt or pants will fit your waist without trying them on. The circumference of your neck is 1/2 the circumference of your waist (and both fluctuate as you lose or gain weight). So, you can hold up the waistband of the garment and if it fits around your neck you are good to go! Try this at home with your favorite skirt and pair of pants so you can get an idea of how you like your clothes to fit.
P.S. - I don't actually recommend not trying clothes on but when heavy-thrifting you can't be weighed down by items that aren't going to fit anyway.