**Literature (old catalogues, fan websites, etc.)**

I managed to date my old Raleigh by finding scans of old catalogues online. That model was only sold in one particular year, but more often you'd get it to within a few years this way. There are many old cycling documents at [Veteran-Cycle Club Online Library](http://veterancycleclublibrary.org.uk/), including plenty that can be accessed for free.  Searching for a brand and plausible year with "catalogue" can often turn up sites dedicated to scans of that brand's literature; you may need to check more than one such site for good coverage.

This will only really work for a major brand that has a bit of a following, as it relies on someone uploading the catalogues. You also need a rough idea (e.g. from the components) to know which catalogues to look in. 

[Bikepedia](http://www.bikepedia.com) is also useful, but mainly for confirming when you think you know a year, and only goes back to 1993.

This would also include fan websites, such as for:
 
 * [Mongoose][1] - uses design features and serial numbers to date


  [1]: http://www.vintagemongoose.com/frame/serials.php