Apart from the weight which basically tells you how big the internal power supplies are you can go by the year and country of manufacture.
You can usually tell the year by the colour and the general design trends which you can spot easily with most mainstream stuff.
For instance 80's- early 90's stuff will be mostly black whereas pre 80's and later 90's stuff will be silver. Analogue tuning on recievers was used in the 70's and before. From the 80's onwards they will have digital tuning.
Famous American stuff like Marantz, McIntosh, Sherwood, Fisher and Empire from the 1950's -60's will often need a major service if you find it in a flea market. Some of it will be slighly hazardous to plug in especially if it has tubes inside. Turntables from this period like the Elac Mirocord are a good buy as they have a Swiss motorboard. Speakers like JBL are also a good buy but watch out for foam rot or budget for reconing. 70's stuff from Harmon Kardon , Marantz / Superscope is very sought after.
European stuff on the whole is also worth picking up. Tandberg, Dual, Lenco, Thorens, Revox, Goldmund, Bang and Olufsen, if you are lucky enough to find any of it, will all be worth a look. It has always been engineered to very high standards and made to last. Philips from the 80's onwards other than in CD players is an exception, as is Grundig, apart from their radios.
Earlier English stuff like Quad, Leak, Garrard, Tannoy, Kef and Goldring has the same caveats as American stuff from 50's -60's. Highly desirable but electronics knowledge is necessary to restore it. A lot of later British stuff from the 70's- 80's is under-engineered so again be cautious. British record players, amplifiers and speakers are the best bet. Linn, Naim, Rogers,Wharfdale, Audiolab...etc
Japanese stuff is more complex but from the 70's and 80's will be the most common I would imagine. The problem here is that whereas most American and European companies are quite specialist and tend to make one or two models every decade, the main Japanese companies are massive and manufacture everything from areoplane parts to elevators. These companies manufacture ranges for different markets so while there may be amazing high-end Panasonic separates (Technics), there are also mass market stuff. To make things even more confusing, in the 70's it will look the same as everything Japanese was over-engineered then. Worse still they also change their model numbers every six months to make it seem like they are being constantly improved which on the whole thay aren't.
Therefore look for the more specialist Japanese companies or high-end marques from the majors. Rotel, Luxman, Pioneer, Denon, Trio / Kenwood, Onkyo and Sansui are names to look for, although Sansui went down hill in the 80's. Aiwa, Akai, Teac / Tascam and of course Nakamichi were all mainly known for tape recording and apart from Teac have failed to remain at the forefront of digital recording. Sony, although massive now were also quite specialist at that time and have always gone for quality markets. Technics (by Panasonic), Optonica (by Sharp), Aurex (Toshiba) are some of the premium brands from larger manufacturers to look for. On the whole this will be superior to the likes of Sanyo, Hitachi, Mitsubishi, Yamaha, JVC and Panasonic but these companies all made very good stuff as well so....
And we havn't even covered vintage headphones....
