There are no general tell-tale signs that hold good always and for every model. The fakes keep getting better, and features that used to be dependable — weight, the fit of the refill, the lettering of the engravings, a service booklet with a stamp — have by now largely been imitated. In the middle and upper price brackets especially, sales photographs often make it impossible to decide whether the pen shown is genuine or a copy.
Security therefore lies less in the object than in where it comes from. Buying from a local specialist shop, from a boutique, from the official online shops and from the handful of authorised online and specialist pen dealers is safe. On auction platforms, check the selling history: someone who has sold pens regularly over the years is a far smaller risk than a seller with no track record in the field. Collecting the pen in person only really helps if you take someone along who knows what they are looking at.
Two rules of thumb still hold: buy only where you can trust the seller, and as far as possible buy only models you know yourself. With older Limited Editions the second-hand market is often the only route, since they are no longer available in the shops; there patience pays, and where large sums are involved an appraisal before buying is money well spent.