The very helpful and knowledgeable
Esmeya had a conversation with me earlier discussing Powersave forced shinies. She is an awesome person and very patient. I'm sure she won't mind if I share parts of what she said to me here, since it is helpful to the Smogon community as a whole.
Here's what she had to say:
"A forced shiny from a player will be marked as being traded and have a different ID/SID in order for it to become shiny. For instance my OT is Halute ID 21913 SID 44725, if I were to force a randomly caught pokemon shiny it would change my ID/SID to make it match a TSV that would make it shiny. If it was actually shiny for my game it would have Halute 21913 TSV: 4018 (as a match) if I forced it shiny it would be Halute xxxxx TSV: [Whatever makes it shiny] it's very easy to tell if it was forced shiny prior to them trading it. Pokemon who evolve via happiness will not be able to evolve, after trading it becomes much harder. Be suspicious of those who offer shinies under their OT but have many different ID's attached to them and always ask for a picture of their trainer ID or ask for their Trainer ID number if you're wary of things under different IDs. (A few people, some I know personally have many save files backed up with the same OT name for different TSV's this is very rare to do but it IS possible for someone to have the same OT Name with a different ID Number combination)."