When evaluating a used dirt bike, you must prioritize the engine condition and compression levels. Check for signs of oil leaks around the gaskets and seals. Inspect the suspension system, specifically looking for pitting or leaks in the fork seals and the responsiveness of the rear shock. Additionally, verify the frame integrity for any cracks or welds that suggest previous structural failure, and ensure the sprockets and chain are not excessively worn, as these indicate the level of previous maintenance.