How do I determine whether a sensor is a thermocouple or a thermistor?
There's one large difference.  A thermocouple is exactly that: two wires welded together.  Measure them while unheated.  Their resistance will be zero (or below an ohm or two, depending on whether your ohmmeter leads or the TC connections were slightly warmed by fingers.)

Small thermistors rarely have values less than 100 ohms.  Common values are 3K, 5K, 10K, or for RTD platinum resistors 1K and 100ohm.  

For non-sensor components, you'll also encounter some fairly big thermal elements with resistance down in the few-ohms region: inrush limiters intended to limit the startup current on banks of filter capacitors.