Measuring speaker impedance dips

How would you go about doing this? Is it as simple as hooking your meter between the positive and negative terminals of your speakers while the system is playing?

If the speaker dips below say 4 ohms what does this mean and why should there be a concern?. Is it that there is very little resistance at that point in time and the amp's signal becomes unstable and less defined?
No, not that simple. You need to be measuring impedance (which includes resistance, capacitance, and inductance) as a function of frequency. This involves looking at voltage and current during a frequency sweep.

A low impedance dip means at that point the speaker is drawing a larger amount of current from the amp. Speakers with such dips below are much harder to drive than those with a fairly flat impedance. Every time the impedance is halved, the required current doubles. Not all amps are up to that, especially down to 2 ohms or less. In general, SS amps tend to do better under these conditions, however, a carefully selected tube amp can be made to work as well.
Only with oscilloscope you can measure impedance dips.

If you think you can simply connect your multimeter to the binding posts of your speaker, you're completely wrong.

What are you trying to accomplish by these measurements?

There are amplifiers that could be stable even shorted for a short time almost like a welding machines with huge power reserve such as Edge, Carvers, Plinius so there's literally no point of impedance dip that could "surprise" most of these amps.
Most modern, higher quality amps are well up to the task loading down to 4 Ohms.

And these measurements are often made with swept signals, not music, which is far more complex and very transient in nature when it comes to short term frequencies of complex harmonic content. With music as the signal source, the amp is not stressed at any one frequency at a time as it is during such analyses.

And there are more parameters to examine following your question, and the excellent responses above; that being the size, configuation and materials of your listening room, the speakers and the power output of the amp. Size of room, coupled to speaker type(s), spec. and amp power will largely determine if you can fill that volume at a desired level, regardless of the impedance minimum or phase angle.

Having said that its always good to know what the output power spec of an amp is at impedance minimum it will feed. If the power needed, along with the speakers and room-to-fill-size is met by your amp, then there is no concern.
What are you trying to accomplish by these measurements?

Nothing really, I have seen this topic many times and have been curious as to it's importance.

Thank you for the responses, it does make sense to me now.
If the company that made your speakers is still in business, call them up and ask for that info. They would have had to test for that.