While there's always good reason to rely on the scale when you need specific amounts, at the grocery store I've found immense value in being able to estimate the weights of vegetable simply by sight and feel. Being able to do this is a skill, one that you can easily develop by first educating your eye, and then putting into practice. Here's a list of what one pound of common vegetables looks like and a few general tips on how to size them up without a scale.
Choose medium-sized vegetables.
This is one of those times that you want to pick the most average vegetable of the bunch — average size, that is. We all have a general idea of what medium looks like and that's what you want if you're looking to pick up a pound by sight. From there you need to know how many of those items you need for whatever vegetable. For onions, two medium-sized onions equal a pound; for kale, it's about five stalks; for a head of cabbage, you want to shoot for the smaller size, with one pound being only a bit larger than a standard softball.
Buy items that are the same size, and use familiar items as a reference.
This isn't the time to pick up one enormous onion and one really tiny one; there's no standard to help you stay consistent. Buy items of the same size for the best, most consistent results. To that same effect, use comparisons that make sense to you. I usually compare different fruits and vegetables to different sports balls because I know how they feel and fit in my hand. They're also universal enough to help most people out as a starting reference. But if you get a one-pound head of cabbage in your hands and suddenly feel like its more akin to a bocce ball, crystal ball, or pomelo, then that's the standard that works for you!
Weigh veggies after you select them to keep your eyes sharp.
I've been shopping like this for a while, picking up total weights by sight alone. I can't tell you how efficient its made me in the grocery store and how capable I feel when cooking. In order to keep my eyes sharp to what weights look like, I will add items to the scale every now and then to make sure what I think a pound looks like is indeed a pound.
Do you shop by weight and without a scale? What are some tricks you use?