How does one check for wear on these plugs?
I'm afraid I've no idea on the mileage as the car is new to our family

 

If unsure, it always a good idea to swap out and start your own maintenance routine.

 

Agreed, and it looks like the Center Electrode is worn down to the point they should be replaced anyway.

Motor On!

 

Im no expert but - I've often Googled on occasion for what a healthy sparkplug should look like; e.g: worn, running rich, lean, oil soaked etc... I change my plugs about every 25k. Given unknown history of your recently aquired Mini; I would say 'When in doubt change them out' and start your own maintenence record.
Congratulations on your new to you Mini.
Welcome to the family!
Oh btw: If your Mini is an S (supercharged/R53), check or have someone check to see if the SC pulley has been replaced with a smaller (%15, %17 reduction) pulley. You will require colder sparkplugs.

 

+1 you dont know the history unless they took it to a place that reports it all the time.