The way Packard had the mechanics test gauges was to use a known good sender. Since the 48-50 used a bimetal heater type sender not easily available today, that is not really feasible anymore. In other years Packard used straight resistance type senders with differing values depending on which brand gauges were used so none are a direct substitution.
There was a poster at the PAC forum who did some testing on his 49 and found some resistance values that worked. His sender was not rebuildable so he decided to convert to a straight resistance aftermarket type sender. As far as I know, his substitution has been successful.
http://www.packardclub.org/forum/viewtopic.php?f=8&t=2302 Unless you have access to a known good gauge to test in your car, you might read thru his posts and duplicate the resistance values he used and see if your gauge responds with the same readings. Without having another sender, that would probably be the best way to test your gauge.
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