These fixed defined values are saved in databases as LAB or (in case of GMG OpenColor profiles) spectral values. The result of a spot color simulation is compared to its corresponding value, and the difference between target and actual value is shown as Delta E. For verifying spot colors, you can either use spot color databases from various suppliers, such as Pantone®, or create your own ones. Please note that GMG ProofControl Inline requires an extra license as well as a printer with integrated measurement device.
Yes, spot colors can be verified. As with pure CMYK ink sets, this is done based on previously measured and thus fixed defined color values.