While there's always good reason to rely on the scale when you need specific amounts, I've found immense value in being able to estimate the weight of fruit 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 fruit looks like and a few general tips on how to size them up without a scale.
Choose medium-sized fruits.
This is one of those times that you want to pick the most average fruit 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 fruit. For oranges, two medium-sized pieces equal a pound; for lemons, it's about four tennis ball-sized pieces; for grapes, you want the bunches to be about six to seven inches long.
Buy items that are the same size, and use familiar items as a reference.
This isn't the time to pick up one enormous peach 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.
Weigh fruits 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 it's 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?