The ease or difficult or removing the valve cover varies depending on the engine design. I think it is a 5 or 10 minute job on a 1999 Corolla, (remove the spark plugs, remove the oil filler cap, remove the PCV hose, remove 4 bolts, and lift the valve cover off) but it could be hours on some other cars.
You can look for gray or black smoke from the exhaust, but modern emissions equipment hides a lot of the stuff that you used to be able to see coming out the tailpipe.
Sludge is probably not visible on the dipstick or in the oil filler neck, but if it were THAT obvious, the engine (and probably the rest of the car) were very badly neglected.
If you had a borescope with a flexible neck, you might be able to sneak it in the PCV or oil filler openings and take a look, but a good borescope costs thousands of dollars.