...How do they know that my NW Rover is not a NE Rover or indeed an All Line Rover?...
Well, they have a few options
1) Not look into it in detail and accept it
2) Look at every detail, think "is it valid or not?" and try to guess - with the risk of getting it wrong
3) Look it up in the manual
Very few guards have memorised the boundaries of all the applicable rovers/rangers and new ones are being introduced, and boundaries changing, all the time.
Equally when using a point-to-point ticket, guards have to decide whether to say they don't think a route is valid, and risk being wrong, or to look it up in the Routeing Guide.
Guards don't like it when proven wrong[1] so if in doubt they usually don't query it rather than risk looking foolish.
Also, a lot of guards are enthusiast-friendly and they see you're on a rover, therefore likely to be an enthusiast. It's like that with PRIV but more so - any glimpse of a PRIV and they tend to accept anything.
[1] A guard who used to do (and maybe still does?) the Chiltern bubble car once got the conditions of a FAM railcard wrong and on a separate occasion tried to say you can't break journey on a CDR, he got so annoyed when proven wrong that he stopped checking tickets and hid in the guards area (slamming the door behind him) for the rest of the journey! He was anti-enthusiast and was annoyed because we were taking photos of the bubble car at Aylesbury. People like him are out there but they are very much a minority.