If you are just using fingerprick testing I think I would be inclined to test maybe 45/30 minutes before, and then again just before, to check direction of travel, if your blood glucose is going up or down. If it's going down, and is within normal range, then you may want to think about eating extra carbs before exercising.
I think it's important to test frequently when exercising. It's probably best to get into the habit of doing this.
But you also need to get to know your own body, and how it responds to exercise.
I'm not a jogger, but I tend to go for quite long, brisk walks, often uphill, so I will often start the walk, when my blood sugar is reaching it's two hour peak, after a meal. Because the exercise will slice off the peak, but after it's sliced off the peak it will start to fall, and can then reach a point where it falls quite rapidly. I often, then have to stop and eat something.
It might be worth experimenting with a brisk half hour walk, and just see how your blood sugar responds, during it, so you get a feel for it, and have a rough idea what it might do.
I would say before, during, and after.
I know this is more testing than you normally do, but it is extremely important to know what your blood sugar is doing during exercise, if you are on insulin, and as you do more exercise your insulin sensitivity might well increase, so you will want to avoid surprises. So just get used to testing before, during, and after exercise.
