The only real way of knowing is to test yourself on an accurate personal breathalyser, for more information see - 

http://www.sobercheck.co.nz/products/personal-breathalysers/lifeguard-breathalyser

This is useful to check your alcohol level BEFORE you blow into your interlock, potentially causing a violation.

Don't use your interlock device as a tool to test yourself so you know what you alcohol level is, this will be recorded as a violation which may potentially extend the length of time you have your interlock installed.