If you can just find a long, skinny piece of metal to poke in a ground a foot or so, you'll know when you hit glass, then you can haul out the shovel. Digging holes kinda sucks, especially it you come up empty after a day of digging. Depressions or abnormal looking 'bumps' in the topography are good places to start probing. Old farms are good places to probe, usually near the foundation as lots of folks had a garbage pit near the foundation. Abandoned cisterns are sometimes filled with bottles and such as well. I've never intentionally went outlooking for bottles, just been lucky to find surface features that indicate bottles below.
