if you're shooting or able to be on set, an X-Rite Passport or datacolor spydercheckr as frame 0 (or with the slate) can work wonders when trying to do a neutral grade. let's you match things up perfectly. the stock co's should really be providing that but probably aren't because it's not "dummy proof" and they probably don't want to bother people with deleting the slate frame.
heck even a plain ol' gray card can at least give you a fighting chance. sometimes the slate has some color swatches built into it. if there's even a slate! (asking for a lot in these days of run and gun shooting, I know...)
Otherwise with nothing else available, it's usually trying to match up something that is a "known" color. Might be a white wall or a swatch something where you can swing it to what it should be and everything else will fall in line, and then you can use consistent grades, setups and looks across the scene with minimal wrenching on individual shots. Hopefully you can find something that supposed to be white or gray in there and get a good read on it, and neutralize it - as in if the red is .24, green is .22, blue is .21, you flatten it out to .225 or something.