I can't see everything but I try hard. I use a Blue Point extending mirror to try to see as much as I can on most cars like the Evap canister on the top of the tank on RAM pick ups and PCV valves under the intake on many cars. I also gently tug on hoses too.
On the later Tundras, I used a mirror and can see one of the two air pumps behind the plastic fender liner.
Good judgement comes from experience. And experience? Well that comes from poor judgement.