ThanksDRoss said:spectrophotometer is the short answer.
There are several ways on the market to profile printersand have different names. On the cheep side is a scanner based system Prifile Prisim $69 the latest software vertion make very acceptable profiles. For $500 or less you can get new PrintfixPro witch will make profiles just as good as the more expensive $1500 units, I am very happy with mine and IMHO it produces perfect profiles. GretagMacbeth Eye-One, and X-rite Pluse are the other options with thier spectrophotometers.
If you only have one or two paper and ink combos to profile send out for them. Many people out there will make them for around $40 each. cathysprofiles.com for one.