You can look at the positivity percent along with the case number.
If the positivity is low for the state, they must be doing some extra testing and outreach beyond just people doing home tests and not reporting.
If the positivity is high for the state, then the case count is going to be low and there's lots more cases out there. Either home testing or just unknown. There's no good way to to know which of those.
Looks like the US is over 10% positivity, so some amount of under count nationally. It's not distributed equally though.
CA 6%
MA 9%
PA 16%
NY (not NYC) 11%
MD 10%
FL 17%
Meaning, the CA undercount is likely less than the PA or FL under count, and the others somewhere in the middle. I didn't get lucky and find one under 5%. I'm guessing there isn't one. If we're lucky, they're all home testing and just not reporting. If we're unlucky, they're mostly people that just don't know and are out spreading to others.