Voltage of a sine wave given max wattage rating
\$\sqrt{30W * 8ohm}= 15.5V\$. Don't hit it with 15.5V right away if possible, i.e. start a little lower and increase.

EDIT: Some clarifications merged here (from my various comments spread around the page) for the benefit of future readers:


The voltage calculated above is RMS. This is usually the case when dealing with AC power requirements; more on how it relates to peak-to-peak voltage etc. can be found at https://en.wikipedia.org/wiki/Amplitude#Definitions_of_the_term 
For a set voltage, the current is a dependant variable by Ohm's law I = V/R. The current calculated from an RMS voltage would also be an RMS value. For devices like speakers, which actually have an non-constant impedance vs. frequency, it's not a bad idea to measure the current when applying a set voltage because the actual impedance at the chosen test frequency may turn out to be different than the single value that gets published/imprinted as the nominal impedance. Some (typically high-end) speaker come with an impedance-vs-frequency curve in their documentation. For others you may find it in some quality review, e.g. it could look like this.
The "music power" is higher than the sine power, twice in this case, because the typical speaker testing standards call for a music-simulating test signal with a crest factor of 6dB. A sine wave has a crest factor of only 3dB, which makes it twice as powerful on average than the 6dB-crest "music power"; a square wave would be even more so, having a 0dB crest factor.