You can't look within the actual sole but you can get an impression of how compressed the midsole is from how the outside of it looks, especialy under the heel. Look for compression lines - though these can be hard to determine between a design feature and actual compression - maybe look at some pictures on new shoe online to compare? Outsole wear can be an indication of how you land, if its not even some bits of the sole may be more compressed than others?
I don't know about shoe life, I suppose the higher tariff shoes are made to last for more miles, so maybe best to go off feel? I rotate my road shoes (and of course wear trail/fell fairly regular) to lessen the impact of wearing one set down on my legs, making them both last longer (I have too many shoes though!). Obviously as you race in very low profile shoes you'll be more susceptible to Achilles niggles etc. than Jonny powder so there'll be more aspect of going off what feels comfortable.
I tend to just go on how things feel. I feel a bit more beaten up when I run in old shoes rather than new. My shoes get relegated to off-road miles for a bit before they finally get retired.
Good tips there Nick thanks - some of them I didn't really think of looking out for actually and yes agreed much less cussioning needed on offroad/muddy trails etc
I agree with out on that Kris, I find my knees more sore when running in old shoes.
Thanks for the tips/advice :)
Might worth taking a picture of your shoes when new and measuring the depth of the visible midsole, noting ot down (on APeven) so you can compare when shoes start getting old.