This article explains the difference between ungrounded and grounded thermocouples.
In the following thermocouples are also sometimes referred to as "probes".
Thermocouples are made of two different metals in the form of wires that are welded together to form a junction. The junction is protected by a metal hull.
When a thermocouple is ungrounded, the junction is detached from the sensor hull and electrically isolated. The detached junction results in slower response times compared to grounded probes. The electrically isolated junction prevents the signal from being interrupted by electrical noise. Thus the accuracy of the temperature measurement is much higher.
At Cropster, we recommend ungrounded probes to measure your temperatures.
A grounded thermocouple is a thermocouple where the junction is welded directly to the inside of the hull at the tip of the sensor which results an effective heat transfer thus faster response times. However, grounded thermocouples are prone to induce noise from ground loops which results in a less accurate reading.
These types of probes are recommended to measure the temperature of gases and liquids.
It is not possible to determine whether the thermocouple is grounded or ungrounded just by looking at it. You will need a multimeter to perform this test as shown in this video by Omega.