The question comes up all the time, so this time I decided it was time to make the reply a short article here.
Despite learning hand tools hands-on as an apprentice, I started in the same boat as most - I want to buy vintage tools but when should I buy and when should I run?
I have bought a lot of tools in throughout my career and I have bought a lot of junk tools in the early portion of it. It will happen, don't be ashamed or embarrassed of it when it does but learn from it. I hope to at least shorten that learning curve a bit for you here.
There really are no clear cut, blanket rules to this especially given the broad range of tools on the market. I don't presume to know all there is to know about distinguishing all good tools from bad tools much less will I try to condense that knowledge into a all encompassing article. The point of this is to give a few general guide lines and I believe it's almost easier to write about what NOT to buy than the opposite. I've not broken this into tool categories but rather that of tool details. I hope this helps to better cement the concepts and my observations in a digestible fashion.
The Brass-en Rule *this includes Bronze too!
In general a vintage tool featuring brass parts will be of an overall higher standard than one with all steel parts. Brass has always been more expensive than mild steel. A very good example of this rule is the hand saw. Generally speaking a back saw with a brass spine will feature a saw plate of higher quality steel than one featuring a steel back. This also extends into the hardware of the saw as brass split nuts are even more indicative of a higher quality saw. There are numerous excellent saws out there with no brass - but poor saws with brass are almost nil.
The Brass-en Rule is also applicable to iron hand planes. Older planes and higher quality planes generally have brass parts. The knob and tote screws for example will have steel threaded rod with brass nuts. For all iron planes a big one is the adjustment wheel. Later and lesser quality planes feature steel and even hard rubber adjustment wheels rather than brass. Despite the obvious yuck factor they are, as a general rule, junk even as a paper weight.
Other miscellaneous tools like squares are also subject to the Brass-en Rule but don't rely on it for chisels - I'll explain further down.
Steel and Iron
For the most part wood workers are dependent upon the black arts of smithing. The number of grades of steel out there these days is simply dizzying! Everything from mild steel (no/low carbon) to powder steel alloys can make it quite a task to comprehend. In terms of vintage tools, forget finding a SAE grade stamp on parts. Also terms such as Warranted Superior are useless and can be translated today as Quality Guaranteed ... by us, the people who want you to buy our products. ie, quite the subjective statement.
What you can rely on are a few different details. First off lets talk about chisels. When buying vintage bench chisels I look at two things first. Is it old and does it have a socket? These two details in combination almost always ensure the chisel is above par. It goes without saying the chisel can still be a poor performer if it's been abused with unscrupulously grinding which can eliminate the factory hardening and temper but the underlying steel will at least be of decent quality. The reason I mention not to rely on the Brassen Rule for chisels is because brass really has no business being on a bench chisel. If there is Brass then it is likely in the form of a ferule - meaning it's a tang designed chisel and we want socket chisels.Again there is an exception to this rule. If the chisel is old enough it may predate the socket design or as the case with vintage mortise chisels they never were made with sockets to begin with. Confusing? I know. If it looks like it was made in a factory and machine ground the rules will apply.
There are many MANY iron planes out there. Any plane made of stamped steel or featuring a stamped steel frog or any stamped parts short of the lateral adjuster is likely to be a later vintage and of poor quality. Even for block planes, a stamped steel body plane is one of those, beyond all doubt, don't think twice, don't even touch it moments - run like a rabbet plane with a badger in pursuit. Stanley did manufacture a steel body plane for a short period of time that were of quality however they were not stamped in a die but cast.
In regards to knowing the quality of a steel in the sense of a working / cutting edge there are few rules to generalize this - rely on the other clues provided to make an educated guess.
This one is kind of fun and in no way an absolute. It's important you know that some excellent quality tools came the factory in some pretty crazy colors so don't let that scare you. What I'm talking about in regards to paint is mainly painted wood. If the knob and tote of a hand plane or the handle of a saw is painted then it's a fair bet the quality of the tool is questionable. As an extension of this, some later vintage planes feature frogs painted painted in odd colors like school bus yellow or highway safety orange. I won't tell you to completely avoid tools based on pain but it should at least be a caution sign telling you to consider the other rules above.
It's not at all uncommon to find planes with replacement knobs and totes and Stanley did manufacture some decent planes with painted knobs and totes, hence me saying this not an absolute.
As with any rule, there are exceptions. By following these rules though you will at the very least not end up with junk. In fact you will have a higher buyer standard than many but it is better to err on the side of quality than otherwise. Now go out, and buy some tools. Follow the rules and leave all of those questionable tools to the seasoned buyer or the trash bin... at least until you learn to distinguish all of the nuances involved.