You're not going to be able to do a full inspection during an open house, the listing agent isn't going to let you climb on the roof, go into crawlspaces/attic, trip off all the GFCI breakers in the house, etc.

What you're suggesting is called a "walk-and-talk" inspection, and is not going to find the high dollar problems a traditional inspection can find.

" ... the listing agent isn't going to let you climb on the roof, go into crawlspaces/attic ..."

I have done all of those things on home viewings / open houses. I've opened septic risers, started generators, tripped breakers ...

Nervous smiles were the worst responses I've ever gotten.

You, and your sibling poster, are correct though: a real, comprehensive inspection is hours long and no amount of brashness is going to bridge that gap - but there is a LOT you can do with 30 minutes of knowledgeable poking and prying. You don't need an hour to see if the sills are rotting or if there are no foundation anchors ... or if all the floor joists are swiss-cheesed up by 40 years of ad-hoc electrical and plumbing runs ...

You can inspect certain things. But you won't be inspecting to see if there's an oil tank buried in the yard, the quality of the chimney, if there's any radiation in the basement, and plenty of other things.

But yes, you can probably inspect the construction quality and foundation pretty easily, a visual inspection of the roof and siding, and if you care the quality and age of utility appliances. You can also pull the permits if you're really interested before placing a bid. You can probably scan for termite damage, etc.

A decent inspection is a 4 hour job or longer generally in my experience.

Checking for radiation in the basement sounds like something you can do passively as you walk through? All you need is a Geiger counter?

I think because Radon is a gas it’s difficult to do. Every test I’ve seen has to be setup and left in place for a few hours.

I understand that - but if you're seeing a lot of houses, it gets very expensive very quickly to bring an inspector to every showing.