You can find this in a few places, depending on how your community is "built."
One of the universal ways to do this is in Admin -> Mod Tools -> Audit Logs.
You can also break it out in CHA, aided with Filters.
CHA -> Content -> Forums -> Members (tab)
Then sort on Solutions Authored and Solutions Accepted. It might not be easy to put it together, but could prove to be a start.
There's also some options in the Advanced Metrics tab in Admin, but I'm having a hard time getting the "Solved by the Question's Author" option to break down by username. I don't know that I've ever seen that data. Hopefully someone else has a hack to share.
Since you're suspecting an individual, then this might help (in advanced):
One more approach will be to check this information in Admin | Mod Tools | Audit Logs.
You can specify the user name of the person and see her activity. If she accepted a post as a solution, you will see this information in comments:
Attr(s) set: "IS SOLUTION", Attr(s) cleared: ""
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