Before you print.

Don’t trust your monitor? Before printing, find out what your photoshop file is really made up of.

The eye-dropper tool and info menu will tell you exactly what your file consists of. Printers will print the information that is in your file, not what you see on your monitor.

ie: if the file has high values of yellow in the mid-tones and highlights, the print will have a yellow cast.

So you can adjust them before you send your image off to the lab for printing.