Other mistake: According to the big photograph in their respective offices Bressler and the DA share the same pair of little daughters. (00:17:05 - 01:10:10)
Other mistake: When Callahan is going to deliver the yellow bag containing the money to Scorpio, he tapes a knife to his leg with sellotape. However, later when he uses the knife to stab Scorpio in the leg, there is no sellotape on his leg.
Other mistake: In the beginning credits of the movie, we see Harry Callahan approaching the swimming pool where the girl was shot. Next we can see some blood in the water of the pool. By the time Callahan gets there, the blood should have diffused already.
Other mistake: Due to the way the opening titles were created, likely with an optical printer on plastic or glass, dust or other particles are very visible when they fade into the shot.
Other mistake: When Inspectors Calahan and Gonzalez, the chief of police and Lieutenant Bressler are planning the nighttime sting, Gonzalez asks how the rest of the group knows Scorpio will enter the one particular rooftop. The chief of police advises Gonzalez that criminals have behavior patterns. In an earlier point in the film, Gonzalez states he has a bachelor's degree in Sociology. Although profiling and criminology often are studied under psychology, he should at least know how people behave and interact as sociology is the study of patterns of social relationships and people.
Answer: He certainly knew at the start, which is why he felt confident in pointing the gun at the (disarmed) robber and pulling the trigger. Harry knew he was bluffing, but passed it off so well the robber wasn't sure. At the end of the film, again, he knew he had one bullet left and wanted to bait Scorpio into attacking him, giving him an excuse to fire.