Agents Sarah Walker and John Casey - one for the CIA and one for the NSA (Don't ask me who's with who!) - weren't paying very much attention to the test being conducted on Chuck by Dr. Zarnow. Had they been paying attention, they would have seen that he was just going through the motions:
Watch the sequence of the slides again, and you'll see that between the dog and the ugly building, there were slides of a hippotamus and of a fat man. And yet they don't show up on that monitor readout.
Zarnow was tipping his hand that he had no intent on checking Chuck to see if he was the real deal or not. Apparently he was already convinced and was just going through the motions in order to find out how best to capture Chuck for his own ends.
Gotta give Dr. Zarnow credit though. It wasn't that long ago that he was a malicious shoe-store clerk - don't believe me? Check out the video for "She's Got Legs" by ZZ Top!