It seems that many would like to prove whether or not the others are Christians.
Some test it on frequency of Bible readings.
To these, I would ask: When is the last time you preached to a complete stranger...on the street? Or, well, since we are not perfect, let me rephrase...when is the last time you feel guilty NOT preaching to a complete stranger on the street?
These are all extreme measures. Another one being:
Take one's facebook history of a year. The percentage of non-Christian related postings will determine one's distance from Christ.