Serato DJ for iOS shows status icons next to tracks in your library to flag anything that needs your attention.

Problem File: 

Serato DJ has detected a problem with this file. It may not play correctly, and it could affect the stability of your library. Try re-encoding the MP3, or remove the track from your library.

Missing File: 

The track can't be found. This usually means the file has been moved, renamed, or deleted since it was added to Serato DJ for iOS. On iOS this also happens if you revoke Serato DJ's access to Files and Folders in Settings > Privacy & Security, or if the file lives outside the Serato DJ folder in the Files app. Re-import the track to clear a missing status. If a missing status and a problem status apply to the same track, the missing status takes priority.

BPM/Beatgrid Lock: 

The track's BPM and beatgrid are locked. You can't edit the BPM or beatgrid for this track while it's locked.

Stems: 

The track has a prepared Stem associated with it.