Comparing sizes helps first of all: the 146 LeGrand is the large Meisterstück, while the Classique, numbers 144 and 145, is distinctly slimmer and shorter. If a supposed LeGrand looks too small, it is usually simply a Classique and not a fake.
For dating, it helps to know that Montblanc reworked the 146 again and again over the decades. Nib, cap ring lettering, trim and above all the feed were altered several times, so the shape of the feed will narrow the year of manufacture down roughly to a decade. A feed of later type in a body that looks older may point to a nib exchanged during servicing, which reduces the collector value but not the value in use.
The second guide is the workmanship: cleanly cut threads, a cap ring seated precisely, even engravings and a piston that runs without juddering. Only Montblanc itself can judge authenticity bindingly, and only with the pen sent in. A 146 found at an antiques market is therefore not automatically suspect, since fakes as a rule follow current models.