I tie a string around the prop shaft behind the spinner and another to the rudder (tailwheel or between hinges) and suspend it from the ceiling. Then I start putting pennies on or over the high-side aileron servo. When it balances I remove that servo and tape the pennies to the servo with electrical tape. If pennies aren't enough I use sheet lead.

Works great with rear-mounted rudder and elevator servos, too.

When building a model I pre-balance it before adding the covering and then the covered balance usually just requires minor additions.