[Uncle Peter]

Apart from taking someone who's mechanically knowledgable if possible:

Do an Experian check on any motor you fancy if it's not a main dealer (they will have already done one)

Online reviews of the dealer

You can often get a feel for anything fishy if you look at the company background and those of the directors on the companies house beta site (i.e. loads of dormant companies or wound-up companies with similar names which may have been used to scam people in the past)