If the mileage has been tampered with then you will see a little red dot by the mileage reading on the dash. Don't know about Gibraltar but in the UK now on the MOT certificate they record the last 4 years mileage readings. I normally check the paperwork for the mileage checks such as MOT certificate, Service record, Bills, receipts etc

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E90 318i 2007 N46 2.0 Manual
E53 X5 2004 M54 3.0 Sport Auto
E83 X3 2004 M54 2.5 Auto