That's a question which has many answers.
Sonus has good advise, I do the same, normally play tracks of a whatever recent material I've worked on, as I know what it sounds like. I also use some old classic CD's that sound good, like some Dire S. stuff.
But - you asked about specific frequencies. If you havn't got the equipment to run tones, Mix Mag sells some very good reference CD's.
Can't resist this one........sorry:
Vox!! I wouldn't play any Toto stuff on principle. And I'm not gossiping, would say the same to their face. Jean-Michel Byron is one of my dearest friends, and as far as I'm concerned one of the best singers. They (Toto) are a bunch of drugged, drunken scumbags and made JM's life hell (he doesn't drink, doesn't do drugs). So he left the band. Ever since then they have badmouth'd him at every given opportunity, in the most malicious, disgusting way. It went as far as ruining a record deal for him, as they convinced the Pres. of the company that he was impossible to work with. Yuck!!!!
In the meantime...... who will laugh last? I've recorded some great stuff with him, just wait and see.