Can the metal body of a dc motor be used as negative terminal for that motor?
The terminals of small, general-purpose DC motors are almost always going to be isolated from the body, because the direction of rotation is determined by the polarity of the terminals, so you want to be able to select that polarity (and thus the direction of rotation) independently of the electrical/mechanical connection of the mechanical structure of the motor.  If one of the motor terminals were connected to its body, and you needed to mount the motor to a grounded metallic structure, you would need a bipolar power supply to be able to change its direction.  It's much simpler to just leave the metallic body isolated and let the user choose the terminal polarity to suit the application.

Motors that are not general purpose are another matter.  If the motor is designed to always run in a specific direction, such as in certain types of pumps, then the polarity of the motor is fixed by the application, and it may be sensible to tie its negative terminal to the housing.  This is especially true in automotive systems, where the frame of the vehicle is often tied to the negative battery terminal and used to carry the return currents for many of the devices in the vehicles.  Electric pumps and such may be constructed this way, however motors that need to be reversible (such as power window motors) will still need to have their terminals isolated.

It would be a simple matter to measure your motor with a multimeter to verify that the terminals are isolated.  Simply check for continuity between each terminal and the case.  Based on the type of motor pictured in the link in your comment, I'm pretty sure you'll find that there is no connection, however if you do, be aware that this does not necessarily mean that you can supply the motor's operating current through the case.  It may be that there is enough of a connection to measure with a meter, but this connection may not be able to carry the full current of the motor without resulting in some damage.