Many people who want to get into the testing business think that they need a whole room full of equipment to start. The truths is that you do not, and you can get away with pretty much the bare essentials. However, you should have a degree in electronics and attend a testing and tagging course prior to opening shop.
A logic pulser
This happens to be the most basic testing equipment you'll need because it helps to troubleshoot digital circuits. The pulser is mainly used to time output and input pulses allowing you to measure the effect of a pulse on a circuit. At times the pulse can also trigger the circuit which otherwise may not be functioning properly. When used correctly pulsers can help to find issues with digital circuits as well as possible solutions.
These help to test the frequency of every signal. Frequency counters help to verify that a circuit is operating the way it was meant to. For instance, infrared transmitters that are required to send a 40KHz signal can be verified if they are in fact sending this frequency out. If this is verified a sticker can be slapped on the gadget accordingly. In the majority of cases the latest frequency counters can be used for an array of digital and analog circuits requiring between 0 to 12 volts. If you want a counter that can detect very high limits then opt for one with a prescaler feature which can ideally be used to test computing equipment.
A Sweep Generator
This works a lot like a function generator but with a slight variation. The gadget can produce an array of signals which are different from what a standard generator can produce. This enables it to sweep for signals starting from the bottom up. However, this gadget can also find frequency issues with an array of circuit types. This means that if a circuit happens to be sensitive to a particular frequency the gadget will detect and inform you about it. This is very useful for testing sensitive electronic equipment that is very sensitive even to the slightest of external interference causing it to malfunction. Testers can suggest that their clients either use improved shielding or a frequency blocker to avoid external redundancies wreaking havoc with the equipment.
Anyone with a degree in electronics will tell you that static electricity can cause all type of problems with appliances. Circuits are often damage and chips can be fried even with the slightest bit of static electricity. The static meter can be used to ensure that an appliance is tolerant to a specific level of static electricity. If the reading is found to be very high then certain steps will need to be taken to make the appliance more static proof but this will mainly depend on the type and use of the appliance.
Getting into the testing business requires experience and knowhow of the equipment listed above. That said once you get a hang of things and you invest in all the best equipment your return on investment is pretty swift.