As a rule, the module depths are listed with the current draws on the module listings themselves. The depths also appear in the 'mouse-over' popups, plus the maximum depth in a given rack gets shown, again, down by the current draws.
Unfortunately, some companies and/or users posting modules are a bit slack when it comes to this measurement...annoyingly so, in fact, because (as you point out) it's a rather critical factor when dealing with typical cabs these days, which tend to be shallower. Cab makers aren't much help at times, either; I had to nag on Arturia earlier this week, in fact, when I couldn't find any listing of the internal depths in their Rackbrute cases (ie: over the busboard and over the empty space). To date, they've still not provided that updated info on their site. grrr......