One way to check, is to compare the heart rates in the first and second test. For example for a ramp test you would expect the second test to have a lower heart rate than the first throughout the test at comparable power levels and then max out at the same heart rate as the first test but at a later stage.
For a 20 min test you would expect both tests to have the same average heart rate.
Sandbagging would clearly show in the ramp test if both tests have the same heart rate build up, with the first stopping at a lower level. In the 20 min test it would show if the average heart rate in the first test is much lower than in the second.