Self-Test includes many different tests inside the unit. Since the unit has a microprocessor, it can send out internal test signals to its I/O circuits and read back the responses. The microprocessor will check the responses against what it thinks they should be and make decisions from that. For example, one simple self-test is to check internal memory by writing to and then reading back from that memory. Another simple check is to generate a simple signal from an I/O port of the micro, feed that to the input of the A channel vertical amplifier through internal switches designed-in for this purpose, and then read the value seen at the A channel A/D converter. Another self check might be to write values to the display sub-processor and then read them back to make sure the write process worked properly. Another example might be to feed the DC power supply rail to the B channel vertical input, through internal switches designed-in for this purpose, and then measure the DC voltage to check that the power supply rail voltage is correct. You can see that if a scope designer wants to, he can think up quite a few different ways to check internal circuits by adding in a few of these switches and by using the circuits that are already handy, like the A/D converters.