There are a lot of spark testers on the market. However I prefer to use high quality units with known values. You can use an inline tester that resemble a test light, but it only takes a few volts to fire it, no where near what it takes to run an engine. Would you consider your coil good after only monitoring it for a few volts? No, just like you would prefer to use an ignition scope each time.
SM Eng: 18 kv
A standard ignition system or non-HEI is capable of producing about 18,000 volts. When testing a coil or secondary ignition component it is important to use a tester that will create this much voltage. Without it, a high tension leak or a weak coil can be overlooked. I prefer using KD tool SM Eng tester, it requires 18,000 volts to fire and is very reliable.
HEI: 28 KV
HEI (high energy ignition) systems are capable of producing 28,000 volts. When testing a coil or secondary ignition component it is important to use a tester that will create this much voltage. Without it, a high tension leak or a weak coil can be overlooked. I prefer using KD tool HEI tester, it requires 28,000 volts to fire and is very reliable.
A bit repetitious, but you need to use the same diagnostic strategy each time you approach an ignition system problem. If you believe the engine lacks spark, and every time you start by installing a tester at the end of the high tension lead, you will get repeatable results and become comfortable in your diagnostic capability.
To use a spark tester, disconnect the high tension lead at spark plug. install tester at end of lead, ground lead to engine block, and crank engine while monitoring tester for spark. Be careful, DO NOT allow your person to come in contact the spark tester or any secondary ignition components for that matter.