1. On a statue of a horse and rider, the number of legs in the air reveals information about how the rider died: both legs in the air means they died during a battle, one leg in the air means they died later of wounds inflicted during a battle.
2. On a statue or grave covering of a knight, the crossing of the legs (sometimes arms) indicates whether they took part in a crusade.
In terms of European history, there is no tradition of indicating on a statue how the individual died, nor how many crusades they went on. You cannot safely infer those things from the stone itself.
Myth and Urban Legend
While Snopes.com claims that part one of this legend is partly true with regards to the statues of the Battle of Gettysburg (and even this might not be deliberate), there is no established tradition of doing this in Europe, although the myth is widespread there: I’ve heard it from numerous sources, as recently as a 2007 book I purchased to give as a present and then thought twice about. The myth of crusading knights is much less widespread, but then again so is knowledge of the crusades.
The supposed logic behind part two is that the crossed legs are another symbol of the Christian cross, a prominent symbol of crusades; crusaders were often said to have ‘taken the cross’ when they went on crusade. However, there are numerous statues of people known to have gone on crusade with uncrossed legs, and vice versa, just as there are riders on statues with raised legs who died of natural causes. This isn’t to say that there are no statues of either type which fit these myths, but these are just coincidences.