It gets harder to plan conclusive tests without dedicated tools on the ignition. A coil is a possibity, but control is also an issue.
A semi critical test can be done by breaking the electrode off a spark plug and attaching it to the coil wire first and then a plug wire. The plug should be laid on the valve cover and the spark should make it to either the threads of the plug or the valve cover; jumping at least a quarter inch gap. At the coil it should be rapid and consistant. At theplug it will obviously happen only an eight as much.
Maddog makes a good point about the starter fluid except that most engines that are otherwise healthy will have no problem lighting off a little (I say little) fuel. The starter fluid is better as it is already atomized. Just its vapors will light right up (make sure that happens inside the motor - starter fluid is even more flammable than gasoline).
Bosch Master, ASE Master, L1
33 years MB technician