As most temperature sensors have an electrical output, that output obviously needs to be measured somehow. That being said, you need to have a measurement device to measure the output, resistance or voltage, for example. 

The measurement device often displays an electrical quantity (resistance, voltage), not temperature. So it is necessary to know how to convert that electrical signal into a temperature value.

Most standard temperature sensors have international standards that specify how to calculate the electrical/temperature conversion, using a table or a formula. If you have a non-standard sensor, you may need to get that information from the sensor manufacturer.

There are also measuring devices that can display the temperature sensor signal directly as temperature. These devices also measure the electrical signal (resistance, voltage) and have the sensor tables (or polynomials/formulas) programmed inside, so they convert it  into temperature. For example, temperature calibrators typically support the most common RTD (resistance temperature detector) and thermocouple (T/C) sensors used in the process industry.