It's relatively straight forward, all watches will have something replaced and the case and bracelet worked on during a service. A brand new Tudor or Rolex will be crisp with clearly defined bevel edges running down the lugs. Over time these can get polished out and that's why people get a bit sniffy about over polishing. You just need to ensure the lugs look right.
With regards to parts, particularly on an older watch people want the dial, hands, insert to be original. The bracelet being changed is not really an issue and the crown devides opinion, if the crown needs replacing to keep the Watch functioning then I say fair enough, but to others they will hunt for a watch that retains its original crown. The crystal will also get changed on older watches.
It all depends on age, a 5-10 year old should be okay, but start looking at 20+ year olds and you have to do your homework and examine the Watch to see what has been service replaced. Not all dealers will give you these details.
If you find a watch, take a few images and post them on here, plenty of people will tell you if there is anything wrong.