To solve the given question, we need to identify what a voltmeter is used to detect. Let's examine the options one by one:

1. Potential difference: A voltmeter is an instrument specifically designed to measure the electrical potential difference between two points in an electric circuit. It is connected across the components to measure the voltage without altering the circuit. Hence, it detects the presence of potential difference.
2. Heat: Measuring heat is typically done with thermometers or thermal sensors, not a voltmeter. Therefore, this option is incorrect.
3. Vibrations: Vibrations are detected using a seismometer, accelerometer, or similar devices, rather than a voltmeter. Hence, this is not the correct option.
4. Magnetic field: Magnetic fields are measured using instruments like magnetometers or Gauss meters, not voltmeters. Thus, this option is also incorrect.

From the analysis above, it is clear that the correct answer is that a voltmeter detects the presence of potential difference.

In conclusion, the primary function of a voltmeter is to measure the voltage or potential difference in an electrical circuit, making "potential difference" the correct choice.