Some of the best people to ask would be diabetics because they usually have to be more focused on what they eat and the possible damage that they can cause to their bodies. - Well, I'm a diabetic. The blood sugar monitors that we get are usually free IF and when you BUY their testing strips - which require a prescription. It is amazing that different foods affect people differently. For me It's pizza and bagels - they cause my blood sugar to go ballistic.
As for when to test, one person said every 15 minutes. The height of blood sugar occurs sometime after 1 1/4 to 1 3/4 hours after eating.

I wish I wasn't diabetic - but I have annoyed my pancreas one too many times by placing too much demand on the pancreas (source of insulin). This was done by not eating 'just to survive' and being overweight.

This may be a tangent - but important information.
I don't know that that is as important as just being mindful to stay away from carbs as much as you can. There is controversy over whether the glycemic index of a food is important. I am sure that there is because the more simple (more processed) a food is, the more quickly the amino alpha-amylase can convert the carb to glucose. The body cannot use energy until it is reduced to glucose (the body has to work harder to convert protein and fat into glucose). Consider onions, raw they have less sugar than they when cooked because the processing (cooking) releases the plant sugars. That's why the "carmalize", which is the sugar browning.