We had to send a photo of the odometer to our insurers - plus mileage is recorded on MOT certs, so easy for them to check. If you are unfortunate enough to have an accident then the mileage will be recorded on the repair invoice.
If we exceed the agreed mileage then our excess increases from £350 to £500 in the event of a claim.
Our ‘new’ van is 9 years old and previous owner had a tracker fitted. We asked insurers for price difference in premium in having functional tracker (ie - yearly tracking subscription paid) v no tracker. Cost of tracking subscription exceeded the insurance saving, so we are insured without a tracker.