Determining exact power being sent to your speaker

How does one go about determining the exact amount of power being delivered by the amplifier to the speakers? Let's say the amp delivers 160w/channel at 4ohms (or so the reading materials state anyway) but yet the speaker specs show 120w maximum. My current integrated amp shows the volume
attenuation (dB units) in .5db steps ranging from -100 (min) up to 0.0 (max).

All that said, the concern is when should I be concerned about pushing too much power to the speakers and how I can determine the "Don't turn it up past this number on the display or you will damage the speakers..."
There are technical experts here who can answer your question but for now, you should rest easy knowing that it is much more likely that you would damage your speakers from driving an underpowered amp into clipping than it would be to overpower them.
The exact power delivered to the speaker can only be measured as transient. It's power at particular moment since we're dealing with the signal and not DC. The specified power is mostly measured within the audible frequency bandwidth. The measured power is usualy done against sine wave that gradually goes from most negative to the most positive value. The gap between maximum values called Amplitude. Power measured against Amplitude is called Maximum Power. The specified power is most-likely RMS power which is product of 0.707*Maximum Power. Sound is far more complexed form of signal that less-likely can be represented by meaning of Amplitude, Maximum Power and RMS. To measure power at particular moment delivered to the load you need oscilloscope. The load is also non-linear and complex value that should also be noted. It consists of linear DC Resistance and complex Signal Reactance...

Done that somewhere in the middle of high school...
Vineman55, average music power delivered to speakers is only few percent of peak power (unless you listen to sinewaves) It is because average loudness you listen to (half of peak loudness) requires only 1/10 of peak power. In addition music has gaps. I would be more concerned with damage to tweeter caused by clipping when amplifier is too weak. Clipping produces a lot high frequency energy (harmonics) that might burn out tweeter.
Thanks for the responses...sounds like I am good to turn it up a bit more than in the past which is good because I was starting to think I might need to get some new speakers...(*grin)
Have you ever owned an amplifier with meters? Chances are you are using a lot less power than you think.