Its not overly stated — at least not in a clear videogamey d&d tag sort of thing– but its usually implied through the “heavy” descriptor in statements such as “this hilt–heavy blade” or “this tip–heavy blade”, so assume that anything that doesn’t have a ‘heavy’ in the text isn’t a heavy weapon. Also assume that anything that doesn’t say “two-handed” is a one-handed weapon. Some weapons can be used one or two handed, and there will be stats for both types.
Thus a Rapier is a one-handed sword since there isn’t any stats for a two-handed mode nor two-handed is mentioned. It’s also a non-heavy sword since there’s no ‘heavy’ mentioned in its description: “A sidesword with a somewhat longer but even more slender blade, tapering for much of its length to a very acute point.“
OTOH, the Falchion is a heavy sword since it’s description goes “An arming sword with a slightly longer hilt and a heavy, broad blade, it’s back straight and blunt and its one edge curved.”. It can be used both one or two-handed.
For instance, the saber is a non-heavy, one-handed blade because it goes “A one edged arming sword with a slight to moderate back-curve.” and because it has no mentions or stats for two hand.
Other than that just advocate common sense or google the swords to see how they are used