There are no other methods to determine BMI as it is the same as how much you weigh. No matter how you work out the calculation, the answer given should be the same otherwise it would be considered inaccurate. Some people suggest adding 5 or so points onto the normal level of a BMI chart for muscular builds, however, again this is not accurate as it depends on how much muscle you have.
I would suggest the best alternative in this case is a body fat measurement. This method is not as straight forward as the BMI as it does not calculate standard measurements to give the result. Instead, a special device is used which measures the body fat. Although these can be purchased, I would advise in the first instance to make an appointment with a GP and ask them to find a body fat measurement. GPs version of these machines will be far more advanced than the ones which can be bought over the counter.
So basically don't worry about BMI showing up as obese, this is likely to be purely a case of muscle weighing more than fat. Only if the body fat measurement shows up as obese will action need to be taken.