It's stamped .223 right on the barrel.......so just go with that as per the Canadian Firearms Safety Course.
If it turns out your chamber is a little on the loose side and closer to a 5.56 well that's just a bonus unless you were looking for a tight .223 chamber? It's stamped .223 for a reason but it might just be acceptable to fire 5.56 just like the rest of the barrels they make.
Maybe shoot it from the hip while testing......