The woman who gave us a tour of the H.G. Matthews factory pointed to a sample wall. "This is our handmade line", she explained. "You can tell, of course, by the beautiful creases in the bricks".
In fact, creases are one of multiple clues that indicate the molding process of a brick. If you ever get a chance to see all sides of a brick, it's quite easy to tell.
Creases show that the original mixture was stiff, not liquid; their direction indicates the force applied, and their unevenness reflects the imprecision of hand-thrown clay.
Check the location and nature of the stamp, if there is one, for further clues. Any brick with a 'frog' and a stamp on its largest face (see here for an example) probably means that a five-sided mold was used.
Bricks made by extrusion are obvious; their extruded sides are smooth and uniform, and most of the time you can see subtle lines from the die. The other faces are rougher, having been cut off. The ones in the photo below also have a continuous stamp, another giveaway.
And these ones? I am getting a lot of creases because my clay has to be pretty stiff. They also often run perpendicular to the opening of the mold because of the way I have to fill the narrow opening (unusual because normally the clay is thrown into the mold, and the creases form horizontally from gravity).