I have to go with the “test” theory. Suspicion is raised when he sees him climbing through the crawl space (where Hans had his own gun), rather than just being in the open or just hiding. Give him an empty gun to see what happens (part of why he turns his back to him). He makes the accent comment after knowing for sure, which was when he hears Hans calling his guys on the radio in his native language right before Hans discovers the gun is empty (why would a panicky worker, attempting to escape, suddenly have a radio and start calmly speaking a foreign language). He would know if the gun was unloaded by the weight only if he was familiar enough with that particular gun.
To pick out the bad guy based on a bit of an accent or holding a cigarette in what is obviously at least a semi-international, Japanese owned, very high brow company seems a bit weird, at least to me. There are a lot of Americans that hold their cigarettes that way simply because it is “Euro” and therefore sophisticated.