I am missing Id tags on f150 differential how else can I determine the correct rear end

 

Welcome to the site.

What year 150? If it's a later build 150, the certification label in the drivers door jamb would have the axle code (if nothing has been changed out)
If correct ratio is in question, another way to figure for ratio w/no tag, is to raise the axle and mark the axle flange location,, rotate the input yoke and count complete turns and partial (clocked) to make the axle rotate a full revolution to your mark.. Depending on MY, axle/gearsets would be a guess...