I try to figure out where things are filmed, too, but know that the earlier the film is, the less chance it was made on location...it was just too hard in those days.
“Expendable” is one of my favorite Duke movies because he really did ACT in it...his face when he hears that Donna Reed is on Corregidor is heart-wrenching...
Other Duke faves are Red River, Stage Coach, Horse Soldiers, Quiet Man and Searchers....thank goodness for the oldies!
IMDB.COM will usually give the filming locations, along with everything else you want to know about, including the age and marital status of the best boy and key grip.