I couldn't find anything for FGH20A15, but I did find this for FCH20A15, which is a double diode. Judging by the PCB, they appear to be connected in parallel to act as a single high current diode.
http://pdf.datasheetcatalog.com/datasheets2/29/29502_1.pdf
It should generally be possible to measure a diode in circuit, unless there is a transformer winding or other component that is low enough impedance to look like a short across the diodes when it is in-circuit.