Grading of notes is actually a very subjective issue as it is an art and opinions do differ from one to another. Serious collectors sometimes do send their notes for a 3rd party professional grader to grade their notes. Notes which are graded professionally normally increase in value as they are verified to be authentic as well as given a grade. One of the popular graders that is being used currently is Paper Money Guaranty (PMG) which is based in the US.
The scan above is an example of a PMG Graded Singapore Orchid Series 25 Dollars, it comes returned in a plastic sleeve with a description, as well as with a grade. This particular note is graded as GEM Uncirculated 65 with Exceptional Paper Quality (EPQ) which is actually one of a very high grade. The highest grade on the PMG scale is 70.