One way of finding out whether someone is stealing your electric by tapping into your electric meter is by turning off you main circuit breaker to the entire house and make sure everything in your house is off including any breakers that might be below your electric meter leading outside.
Then check to see if the meter is still recording power being used by watching that little bar at the bottom.
If it's moving then someone has likely tapped in your electric meter.
To find out for sure you can have either a licensed electrician come out to check or have the electric company come out to see if someone could be potentially stealing your electric.