It’s easy for third edition entries. The date of update is included at the top of the entry, or where additions have been made (but no changing of the original), the additions are segregated to a section at the end of the entry.
The problem is between first and second edition. Without going back to the original print editions, you cannot tell what, if anything, has been changed.
Short of going back to the old editions, the best clue is the date of citations. If all the citations are nineteenth century or earlier, chances are the entry has not been changed since the first edition. But this can be deceptive—some second edition entries added recent citations or senses, but did not change anything else in the entry.
Wikipedia gives a nice tabular summary of exactly when the various fascicles and supplements have been published, from 1888 to the present.