How do they do smog tests on cars?

Smog as in a combination of mist and fog?

@rebbel smoke and fog.

@Dutchess_III Do you mean emissions tests?

@canidmajor Yep, that’s the one.
Thank you!
I was sure it was wrong (I thought that English wouldn’t have been that stupid to make up a combination of words that doesn’t make sense….).
But I couldn’t think of the correct one.

Well that’s what it’s called. What kind of tests do they do?

Most states now use the OBD2 plug under the steering wheel to find any issues with gas engine being out of spec.

They don’t do a “tailpipe” tests in my state.

How do I know if my 2014 Ford Edge has one @Tropical_Willie?

You have one . Virtually all cars made after 2000 have them. If your Check Engine light is off you are ok. (There are lots of caveats but that is generally true.). Of course you still need to have tires, brakes, lights, wipers etc. all in good working order.

Yes, they (OBD ports) were required on all 2002 and later cars.

OBD stands for “On Board Diagnostics.” New cars (including the 1997 Nissan pick-up I had) have the OBD2 system where older cars, like my 1991 Nissan pick-up, have the OBD1 system. My old truck still requires a probe up the tailpipe but newer cars with OBD2 systems use the vehicle’s sensors to read all emission conditions so no probe is required. If your “Check Engine” light is on before an emissions test, the vehicle will fail even before the test facility plugs in the emission test computer to the OBD system computer.
If you have a “Check Engine” light, you have the OBD2 system.

Here’s a link I found: https://www.thecarconnection.com/tips-article/1088310_what-is-a-smog-check.

i.e. they run the engine then sample the exhaust, looking for various compounds e.g. hydrocarbons

Would it have caught the leak in the twin’s dad’s tailpipe?

If the leak was prior to the catalytic converter, they would have smelled gasoline vapors. If it was after, maybe not. They only look for out-of-spec levels coming out of the tailpipe. My step-dad’s car had an exhaust leak at the exhaust manifold. I told him to take it to the dealer, and they found it. I smelled it coming out of the air ducts, so definitely before the cat.