We're trying to get some Google reviews for a company by contacting their past clients and asking them to leave reviews. The process is substantially easier if they already have a Google+ account. Is there an easy way to search by email to see if they have an account? 
How to check if someone is on Google+ by email address?
google plus
In Google Contacts (or through Gmail Contacts) the user information will show a Connected Profile if they're on Google+.