A method of determining the thickness of an internal wall in a gas turbine engine component includes the steps of utilizing flash thermography to measure a complete thickness of a component between an outer wall and at least one enlarged cooling channel at a location where an outer cooling channel is positioned between the outer wall and the at least one enlarged cooling channel and where at least one member spans the cooling channel, such that the thickness is through the member which spans the outer cooling channel. An outer thickness of the component is measured from the outer wall to an outer wall of the outer cooling channel. A thickness is determined from an inner wall of the outer cooling channel to the at least one enlarged cooling channel by subtracting the measured outer thickness from the complete thickness, and also subtracting a known thickness of the outer cooling channel.