By watching when installers are running and comparing the checkboxes presented against a database of "free offer/potentially unwanted program" type material and, if a match is found, automatically unchecking that checkbox.
It is not 100% effective in finding everything, but it's a big help. It also remains under development, so some of these new "techniques" that it does not yet detect for sneaking stuff in during an installs will eventually be detected in all likelihood.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
A little kindness from person to person is better than a vast love for all humankind.
~ Richard Dehmel