The acid test is: take your monster, and take something half the creature's size that's like it (for example, if you saw a 12ft tall giant, you would pick a 6ft tall human). If the smaller thing is significantly heavier than 60 pounds, you cannot levitate the larger thing, because the square-cube law would make that thing be heavier than 500 pounds.
Weight stats for monsters have always been a bit of a crapshoot. Game designers, as it turns out, are usually not very good at biology.
Page 6 of the MM refers us to the PHB for rules on sizes and spaces, but the only relevant section I can find (Movement and Position) has precious little information. So we are forced to extrapolate.
The square-cube law is your friend here. Essentially, when your physical size doubles, your weight is multiplied by 8. You can use this to extrapolate weight of creatures from similar but smaller creatures whose weight is known. You might not know how heavy a Huge Giant is, but you know how heavy a Medium Human is (let's say 150 pounds for ease of math). The giant is therefore 64 times heavier - 8 times heavier for Medium > Large, and then 8 times heavier than that for Large > Huge. This puts the giant at 9600 pounds, more or less. This can vary with different types of giants (a skinny or short giant could weigh as little as 5000 pounds).
You can get more specific if you know the precise height (for example, a 12ft tall giant is only double that of a 6ft human, and if the 6ft human is 200lbs, the giant will only weigh 1600lbs). Still, this means that many Large creatures are unlikely to be affected by the spell.
For creatures with real-life analogues of a similar size, just use the real life counterpart's weight. For example, an Aboleth is a fish, roughly the size of a great white shark. A typical great white is 2000lbs, so we can assume an aboleth is like that too. When your only concern is "more or less than 500lbs?" you don't need to be precise.
You can also try and refer to older edition materials, at least ones where the art makes the monsters look more or less the same. The D&D 3.5 SRD contains weight information on most of its monsters, and it's not terribly likely that a monster would have gotten much chubbier or skinnier over time.