Hi CTNana,
when a Mod edits a post it will how up at the bottom of the post, just like when a poster edits their message. Unless a Mod enters a reason why they edited a post, you will just see a time and who did the editing. But Mods should add reasons for editing someone elses posts. But as far as I am aware, an edit by a Mod should be evident by a automatic time idicator as shown below.
For example
As to the reasons why a Mod would edit someone else's posts:
1) it has broken a forum rule on language, offensive to one or more members, or things of a similar vein
2) to correct a typo in the title. Though spelling mistakes etc are generally left for the poster to correct themselves if they wish to do so, Mods will occassionally correct spellings in thread titles so that it's easier to search for in the future.
3) Fixing hyper-links. Sometimes a link may become broken
I hope that answers you question CTNana. Was there something specific that's occurred here? If you like, you could PM me.