I will admit I may seem like a relative noob according to my find amount (19) but dont let that fool you. I just live in an area with very few caches.
And I have some advice to determine if missing or is a DNF:
1)Check Cache's page first. Read it thoroughly, it may contain inadvertent or blatant hints.
2)Become involved in Forums, there are posts such as "what was your hardest find" or "most unusual find" those may give you some insight into other places to look.
3)Always check within your accuracy range first, then if no find, search within about a 35 foot "radius" because cache hiders coordinates may have been off without their knowing.
4)If there is still no find then check back to the caches page and check the caches log and see how many people ahead of you posted DNF's if there has been a couple or more then I feel it is okay to post a needs maintenance. If there are DNF's sporadically posted then it may be a hard cache to find, and I will post a DNF.