I think it's common sense to ask the seller, condition and their answers will say most. If someone said it had light use, pampered and never knocked about yet it had scratches all over it, a big dent or a high shutter count then alarm bells ring. If it's a hobbyist who photographs landscapes the shutter count could be quite low from their use, a pro wedding photographer it's more likely to be around 100k due to the extra use... though how hard a life it's had is probably more important. I would be less inclined to purchase a camera that's been thrown around, used in harsh weather/environments with little/no additional protection.
You can take a JPEG image and put it through various websites to test the shutter count. Ve recently done this for the d800 I have in the classifieds. I tried it on two websites to ensure the count was correct. You can rely on the image file number as it's possible to reset the count to zero. Also its only a 4 digit count so the maximum number it goes to is 9999 before going back around to 0000 again.
The shutter life quoted by the manufacturer is an average life expectancy. Some may early, some may last longer than expected but most will last around the value quoted. You probably hear about the ones that fail early as most cameras probably don't get get enough use to fail (or even reach a high value) and are 'upgraded' before they fail/reach end of life. If you only took 10k every year (possible with a hobbyist) , most cameras would be expected to last on average 15-20 years! Most don't get any where near this.