Depends which force they belonged to , do some research, some forces have bikes on almost personal issue and in their entire life have only one or two riders and may be very well looked after , others have pool vehicles which are trashed by everyone as there is no percieved ownership or responsibility of care.
If you can find out which force owned them then it is often possible to contact the fleet management and most will tell you it it is a good one or not and also tell you something of its history.
it is also useful as police vehicles if over 3 years of age are tested in house (if the force has its own workshops) and they are NOT recorded on the DVLA MOT system and thus the first time they get a Ministry Test then that becomes their recorded mileage (not unknown for them to lose a lot of miles between the auctions and point of sale ).
Some vehicles are serviced by a Forces own workshops and therefore have no service book or record, others are serviced by the dealer network and will have a full manufacturers service record .
It pays to do a bit of reserch prior to purchase.
Also remember that the wiring may be substantially different from a standard bike and the police are quite happy to randomly knock holes in bits of bodywork to make the crap (sorry Home Office Approved Equipment ) fit.
) Happy days 
