Either put the bike on to the floor so the petrol can come out of the over flow, or take the carb off and theres some screws underneath, unscrew them and take it apart.
Once the carb is apart you should see some floats, if the bottom of the carb is full of petrol, empty it, then put it back together.
If the bottom of the carb is full and the float is at the top then you will know it has flooded.