Two programs that most use are mixed in key or rekordbox. mixed in key can analyse the song and give you the key in one of two formats: Their Camelot format (1A, 2A, 3A etc.) or standard notation. This Camelot format is easier for when you don't know your musical keys and which ones harmonically work well together. However the program costs approx 50 bucks Rekordbox will also tell you the key, however it only gives you the key of the song in standard notation (e.g. E minor, F major etc.) and requires you know which keys mix well together (e.g. F minor and C minor mix well together harmonically). Hope this helps and I haven't confused you more.