Instructions
Look at the model to see what kind of vehicle or machine it purports to be. If you're holding a model of a car that was on the road 50 years ago, there is a good chance that the model dates from the same period, because manufacturers tend to produce models that keep pace with the times. Although it is true that modern manufacturers do produce lines of "retro" vehicles, this is still a worthwhile starting point.
Inspect the model to see whether it has a windshield and a detailed interior. These denote a model that dates from no earlier than the 1960s.
Scrutinize the quality of the casting, looking for any blemishes or crudeness that compare unfavorably to modern toys. Diecasts from before World War II were often made with lead, which doesn't retain as much fine detail as zinc alloys. Some of the early zinc alloys have also deteriorated over time. Both of these are signs of an early diecast model.
Look on the underside at the maker's stamp. Beneath this should be the country of manufacture. The exact dates will change from company to company, but in the 1980s it became common for manufacturers to shift production from their home countries to places like Taiwan and China. Therefore, a diecast model marked "Made in Great Britain" is likely to date to the early 1980s at the latest.