I need some advice for my 1996 Mustang GT?

Ok tomarrow Im having a compression check done on my car due to the fact that I keep getting a mis fire on cylinder #5. I am trying to rule out a bad piston ring and hoping its just a bad valve seal or head gasket as to why I am getting oil on the threads of my spark plug. I have done every thing to rule out the electrical system being bad. Now what can I do to tell that the mechanics arent trying to pull one over on me. What is the normal compression so that when they do the test I can tell what they are saying is the truth. Its the whole concept of if its not broke dont fix it. Please help.