Its become difficult not just because of the miles but because its a 16 yr old car. For instance, Kirban's personal vehicle had over 130k on it and its interior was perfect. My car's interior was perfect when I got it at 39k and its still perfect at 60k.
But check the steering wheel for wear - the carpet especially around the driver's foot. The sag or lack there of in the front seats. The headliner's firmness along with the visors.
Look at the jack.
The paint may have swirls, etc, but at 40k you'd hope its not looking like the paint has been 'rubbed' off, you might see that later at 140k.
Find out if the valve springs are new - you might have a better chance of gauging the mileage by how the engine feels. Check the condition of the gas tank.
Best thing is to see receipts. Normally someone with a 40k 16 year old car has taken care of it and should have most receipts.