A machine and a method for assembling a vehicle body is disclosed as including a locating pin (26) formed with a seating flange portion (35) having a work seating surface (35a) and a clamp arm (39) contained in the locating pin to be operable in a clamping or unclamping movement responsive to an extrusion and retraction of a clamp cylinder (34). A detection pin (47) is mounted for an extruding and extracting movement from the work seating surface (35a), with the detection pin (47), a shaft (44) ad a proximity switch (48) forming a work seating detection mechanism (49). A mutual engagement between the locating pin (26) and a locating bore (R) allows a panel W3 to be positioned, simultaneously detecting the presence of seating of the panel W3 responsive to a movement of the detection pin (47).