This LED was sold to me as 5 Watt LED, what's its voltage rating?
Instead of asking, measure it.  You already know not to exceed 5 W.  Hook up the LED to a variable power supply, maybe with a 1 &Omega; power resistor in series.  Measure the voltage and current as you slowly crank up the output of the supply, then you can see directly what the voltage and current is at 5 W.

Note that this is, of course, all in the LED's datasheet.  Unless you are getting a really good deal and are willing to live with the consequences of not knowing the specs or that the part might be counterfeit, don't buy something without a datasheet.

Be careful about running this unit at more than a watt or two.  It most likely is only specified for 5 W at a certain case temperature that can't be achieved without significant heat sinking.  Forcing 5 W through it just sitting on the bench will likely destroy it.  Of course, without a datasheet, you don't know how "embellished" the 5 W figure from the seller is.  Nobody would buy surplus 3 W LEDs and resell them without the datasheet as 5 W LEDs.  Nah, that never happens, no need to worry about it.