Botox Dilution: How to Be Sure You Get the Units of Botox Paid for

As you know Botox dosing is measured in biological units, not weight or volume. An honest provider will give the number of units you are paying for regardless of dilution.
Here are a couple of things to keep in mind: Although the on-label dilution uses preservative-free saline, most experienced injectors use preserved because it hurts less and doesn't seem to affect the potency.
Some experts also use different dilutions for different areas to try and control the spread more precisely, though it is debatable whether this is useful.
The recommended dilution on the label is based on the preclinical studies and so the FDA mandates that the label say that; however, adjustments based on clinical experience are the norm.

Botox dilution has nothing to do with the number of units received and dilution amounts will vary depending on injector preference. We will say that in our experience if you are being sold Botox by the "syringe", then you can almost certaily assume it is being overdiluted or diluted at the high end of the acceptable range.
With regard to how you will know how many units you are getting. I am not sure you will ever know exactly. But regardless of whether your injector is board certified, there are certain medical practices in any area that have a good reputation with lots of referrals and other practices that do not have a good reputation and get very few referrals. The best way to find an upstanding practice that injects what you pay for is to ask around for a practice with a good reputation.

Your best insurance for having a proper dilution of Botox is to go to a top plastic surgeon or dermatologist and have your treatment.
Their reputations are your best guarantee of having the procedure done artistically and effectively.
There is no way you can supervise how much saline a doctor is putting into a Botox bottle. If you are even asking this question, you are in the wrong office.
If you go to an unlicensed practitioner or think you can get top notch treatment for $10 a unit in a spa, or get your Botox on a table with stirrups, be prepared for droopy eyelids, facial asymmetries, short acting Botox, frequent "touch ups" and eventually resistance to the effects of Botox.