Other than building the firmware and flashing it yourself not really?

Usually they will have flashing instructions somewhere, and thats a goodish sign. Even better if they have the source code published somewhere. But unless you build it and flash it yourself its impossible to know what is on there.

For a real-world example, many Anet A8 machines were shipping with “marlin”, but with the runaway thermal protection disabled.

but they can advertise other things that aren’t a direct conflict of interest.

Advertising things that arent a direct conflict of interest just results in poorly targeted ads, which both consumers and advertisers don’t like.

On the “open source” side, its not enough that the firmware is open source, the flashed binary needs to actually be unmodified. Marlin and klipper run 99% of the budget 3d printer space, and both are open source projects. There are still dozens of printers that are shipped with modified firmware with the changes kept secret. GPL is only as powerful as your lawyers are.

For a similar tool for windows, there is: https://www.grc.com/wizmo/wizmo.htm

Even when not logged it, yt builds a profile of what you watch. I’m not logged in on my work browser, but it still knows what I watch frequently. So this ones on you…