The Automatic Sector Fix command - alt+F10 - can resolve many common Sidedef sector reference mistakes automatically.
1. Select ALL the Linedefs that you want to check. Usually all these linedefs form an enclosed area, that is commonly referred to many as a "sector". However, that is misleading. Please refer to A detailed look at WAD files for a discussion about the term Sector.
2. Now press alt+F10 and any fixed references will appear in a list.
Not all errors are not fixed, mainly those with opposing sides that were incorrect to begin with. The opposing Linedefs (that were NOT selected) must have the correct sectors.