Discussion of implementation
You can do this in a variety of ways. I would need to go investigate more about the current implementation of some of the game featuers to give a more informed opinion about how this could be implemented.
From my current position and with the information I have. I would propose what I currently think would be a seamless implementation by adding an option for "ranked/unranked" and this option is selectable by host, and forces its application onto all clients in the lobby, this will disable the UI for UI mods and all currently selected mods are also disabled. Whitelisted UI mods are those integrated into the FA client already and any future mods that want to be integrated.
If your question is more about enforcing this to avoid people replacing integrated mods with their own cheated version, then I would need to look more into the implementation of the the integrated FA mods already. I suppose verification of the UUID might work, or have each UI mod on load send some data to the server, and verify its there, and if the data is not received by the server to disconnect the client, or raise a flag to moderators/admins that this is a potential cheater.
As a further comment to those who simply say "cheating doesn't exist" provide evidence. We do not currently have a definition of what is cheating.
Example of gamebreaking information modification:
If i make a mod tomorrow that automatically pings any t3 bombers as soon as they are in vision of a radar, even if the models are physically not loaded, that gives me a competitive advantage. In fact if I made this mod I bet it would be very popular to those who take this game too seriously. I'll call it the "Anti-Snipe" mod. It will ping the location of any beetles/mercy/T3 bomber/T3 solace detected within a certain radius of any ACU, and then send out a message into allied chat "WARNING SNIPE".
Obviously, this would drastically decrease the effectiveness of ACU snipes. It would actually ruin a complete part of FA gameplay.
Is it cheating? Well technically no not under the current definition as many other information mods are allowed and are not "cheating" per se.
As to evidence of cheaters? If i make a mod tomorrow that shows me the income of enemy or hell on the "mass" overlay incorporated other information about different units, this mod as of current is impossible to detect by observers or opponents and gives me an advantage. With such a basic observer function in FA and a huge amount of complexity when it comes to gameplay, it is very difficult to assess if people are cheating or not, unless they are overly blatant about it.
Why is cheating so problematic?
When a cheat becomes ubiquitous among certain users that have influence, they defend said cheat as a "normal function" of the game and deny competitive advantage. This leads to nothing getting done about it.
Auto-eco mods that auto pause engineers are no different from recoil macros in an FPS game.
Information mods can be no different that wall hacking (seeing location of all enemies) in an FPS game.
Essentially, there are number of users, many of whom are high rated and play in tournaments on a regular basis, who use these said cheats, and they find it offensive to tell them that they are cheating. Because they assume everyone uses the cheat they think it is morally okay.
On your question to do with evidence, please read above. It is physically not possible to detect non-blatant use of UI mods that are cheating. Which is why I favour a whitelist, as then it puts everyone on the same level playing field. The mods are integrated so that everyone has the same information and options.
On your second point, I would argue that any competitive advantage is cheating. If I install modification that allows me to select or configure or macro my units in a way my opponent does not have the same option of doing without installing that mod, then that mod is a cheat. The argument "install the mod" is facile. The same logic could be used to aimbotting and recoil macros in CS GO for example.
All players should have the same base starting point functionally wise in the game. If you have to install something which is external to the base game to be put on a level playing field then that modification is a cheat as it means one player has a competitive advantage over another. Mods are genuienly useful and provide QOL fixes can be implemented (as many already are) through a whitelist.
If the game is not-ranked then people can use whatever they like, so we're not squashing or restricting creative talent in the map and mod making process. This will also mean that UI mods have a testing environment before they are integrated.