This is the clearest case in wine of a term that means a great deal on one label and nothing at all on another that looks identical. The country of origin is what decides which you are holding.
Spain, where it is defined
Spanish law sets minimum total ageing and minimum time in oak for Crianza, Reserva and Gran Reserva. The requirements differ between red wine and white or rosé, and individual denominations may impose longer periods than the national floor — Rioja is the best-known example. So a Rioja Reserva has met a specific, checkable standard.
Italy
Riserva is regulated per appellation rather than nationally, so the requirement in Chianti Classico is not the requirement in Barolo. It always means additional ageing relative to the base wine of that same appellation, and never a fixed number across Italy.
Everywhere else
In most of the New World, including the United States, "Reserve" carries no legal minimum whatsoever. It may indicate a producer's genuine internal selection, and it may indicate nothing but a different label. Nothing on the bottle distinguishes the two cases.