Generally speaking, the average person cannot watch a movie being filmed unless she has special permission or are somehow involved in the production. The rare exception is a film that happens to be shooting in a public location that is not completely closed off to the general populace.
Friends and relatives of cast and crew often set visits simply to observe a movie being filmed, but without one of these special relationships, the possibility of being an on-set spectator is almost non-existent. Occasionally, film productions shoot in public places out of necessity. It is theoretically possible in these cases to witness some of the on-location action. However, these situations are usually tightly controlled and configured so that spectators are kept at a substantial distance from the production activity.
As of 2015, challenging locations such as the streets of New York City or the Las Vegas strip are more likely to be created via computer-generated imagery as opposed to the time and trouble involved in attempting to capture the real thing, making the chances of catching filming in progress less likely than ever.