(you would want me on your team for Trivial Pursuits . . .)
I noticed a page with a chart of 'stamps' - apparently Snap-on used that method, similar to silversmiths, for dating.
http://www.collectingsnapon.com/Data_Chart/Date%20Chart.htmlPresumably (from reading comments in the forum) the stamp is usually located after the 'USA' on the handle of the wrenches - or on the outside of the sockets . . . I'd imagine it is in similar locations on other tools.
Having said that, there is a rather involved discussion about why some of the tools do not have a datestamp - something to do with the one-year warranty the company offered. In the conversation, the person indicated that a stamp of "G" or "E" was common during WWII, and indicated that there was no warranty on the tool (government or emergency is what they stood for).
Whatever you do, don't scour the metal with a pad or abrasive or polish - that could make them less attractive to potential buyers. If you find something that might be a stamp, try soaking the area with a little vinegar on a cotton ball and see if it will lift the crud enough to read it (found that out the hard way when I thoroughly cleaned what turned out to be an 18th century silver plate spoon. Fortunately, it's a pretty spoon and I didn't really want to sell it . . .)