A two-winding transformer must have at least 4 wires, an autotransformer need only have three. So, if the device has only three connections, it cannot be a two-winding transformer. Or, if it is, it is internally connected as an autotransformer and is treated the same. On the other hand, if it has four or more connections, it could be either, because the manufacturer could have internally brought out the common wire to two connections to make it easier to wire the line and load. You would have to measure with an ohmmeter to be sure! Warning! Do not attempt to make any such measurements unless you are trained and fully qualified to do so. If it is a large transformer used in a power distribution system it will have this information on the nameplate.