How is it that you guys find out about "digs"? This is the first time I've ever heard of people actualing going to a place and digging for old bottles. How do you know there are bottles there? Sound very interesting.
Research is good, like diggerboy mentioned. Local library and historical society is a good place to start. Plus-just looking. I took my nephew out to a lake to look for 1970's No Depsit No Return soda bottles. (easy to find, usually above ground) Trying to get him interested in digging and bottles. We ended up finding an old stone foundation of a home. Saw a lot of broken older glass. Just started poking around and found the trash pit. (Usually a sunken place close to house) We found stuff from 1890's to early 1900's. Then, found a ravine close by and it is full of stuff we hadn't even been able to dig yet.
Check for a local bottle club too. Nothing like talking to an old digger for great tips!
Good luck!
Sanborn maps are a help also ! And just talking to older long time residents of an area can sometimes lead to a site . And a lot of probing in a suspected site.
Brian