I would say the best way to deal with it is to put it right out on the table when you discover it. It's on the individual who is covertly dishing it out to own up to, accept responsibility and explain.
Just remember that you personally cannot make someone be honest and forthright if they don't wish to be. You can call an individual on their behavior but in the end, it's up to you how you will deal with it. One must be prepared to hear denials and excuses which may be thrown back at you. Calmness on your part would be the best way to go through it. Being prepared will also help you keep your calm and help you decide where to go from there in the relationship.
Wishing you well!

sabby