Red flags for me are :
First thing built is an MG sentry.
If an engineer demands deposits(esp as the host) even from classes who benefit more to use the power on perks. Personally, I just find they either do this to build unneeded stuff or to compensate for not being very good engineers to start with.
If randoms start building stuff when there’s an engineer present and/or don’t stop with it when told not to(or if they’re not kicked by the host). Usually they’re the same ones that will upgrade your barriers from level 1s or 2s to level 3s or 4s as soon as they get an opportunity to.
Those are the things I can think of right now with my brain operating in half sleep mode.