There's no 'best' time to test your ketones as the level fluctuates throughout the day and depends on how long you have been in a fasted state, whether you have exercised and whether what you ate last contained lots of carbs.
Generally in the mornings you'll have a slightly lower level and higher in the afternoons.
A good indication on how you're doing is to test before you break your overnight fast. You should be showing above 0.5 on the monitor to be in Nutritional ketosis and from about 1.0 to be in optimal ketosis. Another good test is to see how your levels have recovered 2 hours after eating 
The monitor I use is a dual blood monitor from Spirit Health Care: Click here to visit their website
I have no affiliation with Spirit health care and my view is entirely based on my use of the monitor over the last couple of years. All ketone monitors require their own brand of strips, however from our own research they all seem to be around the same price. Click here to watch a video on how to use the monitor.
It isn't necessary to test ketones daily or even weekly!
One tip is to order extra blood and ketone testing strips with your monitor as it only comes with a couple to save on additional postage costs.
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Exogenous Ketones
Exogenous ketones were created to help with neurological conditions, however we have seen these be promoted as a short-cut to achieve ketosis or lead a keto lifestyle.
If you're looking to lead a clean lifestyle (and you're not looking at these for neurological issues), then there is no benefit to using these. See this article by Diet Doctor for more information.
