A little thing called socialization is the big key here. By socializing your puppy properly to all kinds of appropriate people and places, and your puppy having positive experiences during socialization, your pup should grow into a dog who can tell the difference between who is right and who is wrong. As for training him to alert, thats something they either do or they dont.
Shasta GSD 5/5/10 CGC, ITD, TC
"Dax" Thor z vom Weberhaus BCAT CAX2 CGCA ETD HCT NCO-1 PKD-T RATI RATN RA TC TKA 3/18/2013
"Hades" Guilty vom Blutfeuer 10/26/2018
Zena GSD 6/1/03-2/16/2016
Riley GSD/BC 1/10/05-2/1/2013