I'm confused as to why you should be "going for" such a test. A post-prandial, i.e.. after meal, test, merely tells you how you have reacted to a particular meal and only makes sense when compared with the pre-prandial, or before meal, test.
It is more usual for doctors to ask you to have a fasting test or a glucose tolerance test. In both cases, you should not eat for 12(?) hours before the test. A glucose tolerance test reveals your reaction to a measured and standard amount of sugar and is carried out in a resting situation and, therefore the results are meaningful and comparable with recognised standards. Any normal meal won't do this. A normal meal's effect on your sugar levels will be influenced by exercise and fat content of the meal, as well as the sugar it contains.
Sally