You might check someone's account age or history first. Typically people won't accumulate many thousands of karma points if they are shilling for brands.
I've been active here for over a decade. If I did that so I could start shilling for brands in my 40s, that'd be a hell of a long con. Also, I pointed out that it's useful to check users' histories. A few seconds with my comment history would reveal that I'm more of a grump than a shill.