As a proxy in CareerMode I just keep an eye if the “Press Y to rewind” hint appears.
If it does the lap is compromised, and no longer clean.
Does it do that with Rewind disabled?
To me they are overthinking it. Mark a dirty lap if a lap is dirty, withstanding rubbing and drifting in all races. If a mod was used, the lap was by default, dirty. Its not that confusing. This is simple stuff that makes me scratch a bald spot on my scalp.
If I ran a clean lap, I would like to know it, regardless. Knowing how well I’m doing during any race, escpecially with the new tracks, helps me improve and try to repeat what I did right, and avoid what I did wrong. I want every race to matter.
It’s also a small achievement to get thru 24 ‘drivatars’ and have a clean lap. Further, perhaps we should be rewarded for said clean lap with 24 drivatars.
There is a simple solution that will unfortunately have to wait until FM7: Remove mods from the game (or at least give us an “off switch” to exclude them from level spins) and replace them with the dirty lap indicator 
Pretty sure the fix would be to add the dirty indicator to career. Can’t imagine it’s an impossible thing.
It really does help for getting to know these new tracks, their boundaries, etc.
What really helps in learning a track is the Rivals Challenges. They are the place where you should be worrying about clean laps, not in a race where 99% of the time your lap is dirty unless you are in 1st place for an entire lap (since drafting causes a dirty lap).
They could also add a mod logo to the LBs instead of dirty READ DIRTY lap. It’s not a dirty lap, it’s a MODDED lap. omg.
I’m sure there were plenty of solutions to this, which is why it’s one of the more mindblowing oversights in my mind. There are always factors for things we don’t know, maybe there’s some deep reasoning behind this one … but gee there are a lot of these small things that are like WOW.
Obligarory Great game tho.
+1
I would really like to see this added, I miss it.