All you need to do is be open. Do not lie for money on the internet or mail. State you work for other companies, state you are on other boards, etc. When you lie to people for money is where things go really bad (at least in the US).
I'm very careful about things I do on the side that are related to my day job and I'm very public with my management. In general, those things are seen as positives. But if I created any heartburn, I would generally pull back--or escalate it if I felt strongly.
I'm not going to take a big contracting contract with a potential competitor without discussing it with someone.
Why one should disclose that? And how and why that's fraud? Do people in US disclose things like 'I flip burgers during the day' - 35 hours/week and also 'work as an office cleaner on weekends and evenings' - another 35 hours/week. Technically those are two times full employments. I'm not from US and may lack some details how it works there. My example above it not theoretical I know person who works like that,non-US location.
Those are not conflicts of interest. (Or shouldn't be.)
Working "full time" for Google and "full time" for some startup in a related area probably is. (If the startup is an unrelated area related to medicine maybe but I'd still expect both companies to be aware of the situation.)
(wire/mail fraud 941. 18 U.S.C. 1343).