I'm not sure how you check to see if it is shimmed correctly but I'm pretty sure the cam gear is pressed onto the end of the camshaft. Then there is a retainer plate behind the cam gear where the bolts go that actually hold the cam shaft in. I think the shim that determines end play is behind that plate. Thinking of that arrangement it could only be the shim is missing or wrong size or the retainer plate bolts could be loose letting it slide too far forward.
The cam may not wiggle easy while it's installed due to the pressure from the lifters and valve springs.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang