Usually the cab drivers honk anyone, who looks like they are looking for a cab. Let's say you are waiting for a friend to pick you up with her car. You are standing by the road. The cab s are going to slow down and honk at you. Some of them will even stop and look at you like "Why aren't you taking a cab? What's wrong with you?".
However, you may be standing by a very crowded sidewalk and see the need to stop a cab. Then, you should raise your arm with a 45 degrees towards the road and yell "Taksi" [tuck - C]