I haven't kept very current on piston engine work, but I do recall pulling the valve covers, rockers and pushrods, then fishing the lifter/cam follower out so that the cam lobes are in view through the pushrod tubes. I don't believe this is possible on all engine models though.
The only engine that you can do this on the Cessna 172 is the H2AD. In fact, they designed the engine specifically so you could remove the lifter bodies without splitting the case. Every other 320 will let you take the lifters out, but the lifter bodies are stuck in there.
Ironically, it was the smaller lifter bodies that were the Achilles Heel of the H2AD. With less surface area, the surfaces were loaded more and contributed to this engine's terrible reputation of eating camshafts.
So yes, you will have to pull off two jugs to inspect the cam lobes and lifter bodies, which is about a day's work. CAR 625 APP C says you only need to do that if the engine has been out of service for 12 months. A better way might be to dump the oil and run a magnet through it and split open the filter (a bad cam will make metal show up in a new filter within five minutes) and use a bore scope for the cylinders.
450tt in 15 years is useless information without other qualifiers. If the plane lived in a dry climate, in a hangar, and was run for at least an hour every time it started, with preheat if necessary, and frequent oil changes regardless of hours, it would probably be one of the better engines out there.
I just finished inspecting an engine (IO-520-F) with 330 SMOH done in 1995 that had not been run in eight years! I bore scoped it, did a leakdown, put new oil and filters on it, new spark plugs, drained the old fuel and checked the sumps/screens, ran it for a while, checked the governor screen, then got a ferry permit and flew it to home base.
Pulled three jugs off and.... nothing. No rust in the cylinder barrels (plain steel), no corrosion on the pistons or cylinder heads, no rust spots on the cam, crank, or con rods, no pitting or spalling on the cam lobes or lifter bodies. Just some minor scoring of the cylinder walls and pistons (you could still see the hone and machine marks through them) and a bit of irregular wear on the cam... which is just about any engine out there with a few hundred hours. Oil filter came out clean.
I chalk it up to the dry climate it was in, the fact that it was hangared, that it had clean oil in it (not preservative--which would have been better), and that the owner didn't run it at all in the intervening time. The plane needs to fly, not just run.