You can tell because a single cash box or wad of cash from any downed enemy is consistent with your character level and mayhem level. These “wads” as I call them, can come either singly, or in multiples of 3.
With no loot modifiers like those found on artifacts or character builds or whatever:
At level 57, with no mayhem level, that will be 3194 or 9582 (3194x3).
At level 57 and mayhem 4, that will be 5749 or 17247 (5749x3).
At level 57 and mayhem 6, that will be 7026 or 21078 (7026x3).
Note that it is possible, when killing many enemies or bosses or other “chaotic” situations, cash wads can also drop as a single bill ($1) or stacks of these grouped together as the system apparently tries to keep up or wads drop close together or whatever. I can’t control all that adequately to suss out all the rules involved, but can easily and consistently replicate looting a single cash box or picking up a drop from a single downed enemy…and these always scale with the character level, mayhem level, and whether or not you have gone down at any point during the current instance of that map.