How do you easily tell if a CFL bulb is bad?

I have found that the cause of death of most of my CFLs has been cheap capacitors that fail prematurely. This has usually caused too much heat buildup in either the capacitors of the FETs they are buffering, leading to a catastrophic (read, sparks and smoke) failure. As a result, if there is a question about the condition of a bulb, the first thing I do is give it the "smell test". If it doesn't smell like charred electronics, there's a decent chance it works.

"Warning: When replacing a CFL bulb into a light socket, do not hold it by the tubes, particularly if it is the type with two or three U-tubes coming up from the base !

Just a gentle squeeze will crack them !

Always hold it by the base when fitting it in the socket, do not apply any pressure to the tubes."

"The few CFL lightbulbs that I have seen dead had different visual symptoms.

One had black rings at the base of the bulb glass. It looked like a regular florescent tube with dark ends as it goes bad. If memory serves correct, this was a high duty cycle fixture - it turned on and off a lot.

One had significantly yellowed plastic at the vent between where the two ends of the glass entered the base. It had a distinct burnt electronics smell too it. The newest bulbs may no longer have this vent.

The third type of failure I have seen was a catastrophic failure of the glass bulb. I turned the light on and the glass jumped right off of the base. I think it had been struck and had cracked. Turning the light on finished the job and the bulb broke in half. There is a mercury exposure risk to this type of failure.

I usually put a light into a small clamp-light to test used bulbs before putting them into a difficult to reach fixture. They have such a long life, I've broken more than I've had burn out."

"With the variety of possible faults this is not going to easy if not impossible. The simplicity of the Filament bulb allowed you to see a broken or burnt out filament. The CFL can have quite a few components fail without any external signs

sorry :)"

"When in doubt, lamps should always be replaced, because old lamps continuing using the same amount of energy, but put out less light.

Well maintained lighting systems always replace lamps before they burn out."

I would test to see if it completes a circuit using a voltmeter or a wire-battery-led setup.