Your basal insulin should maintain a steady (flatline) BG for you indefinately without the additon of any food or exercise. The common way to check your basal rates is to skip a meal and check your BG every hour or two until you eat again. You are correct that having Insulin on Board is not appropriate when doing basal tests. What type of insulin are you using and how long is it active. I am on Humalog and it is active for a little under 4 hours.
While fasting and doing your basal testings, check your BG at least every 2 hours. Your BG should stay within +/- 30 mg/dL of your starting value. If your BG does not,then you need to consider adjusting your basal rates.
To adjust basal rates you need to know the peak activity time of your insulin. My humalog lasts 4 hours and peaks at 2 hours. So if my BG starts climbing at 6 pm. I would adjust my basal rates up at 4 pm. I would recomend small changes to your basal rates, because they can have a bigger impact than you may guess. The books than say to do the basal testing for that period again to see if you adjustments are spot on.
In the real world I would recomend that you skip one meal (breakfast, lunch, dinner, or evening snack)and basal test that period. When you feel comfortable with that time slot then tackle another time slot. I find a CGM to be a great tool for basal rate testing. Good luck with the overnight hours as those are a pain!