How to know if an airport has PAPI lights?
Whether an airport has PAPI lights or not is indicated on the approach charts. For instance on this chart, there is a schematic of the airport at the bottom of page 3 that shows you where the PAPI is on the airfield. 

If you are looking for airports in France, you can find all visual approach charts on the government website. 
Otherwise, this wiki has various links to approach chart ressources.

Edit: If there is not indication concerning the location of the PAPI on the approach chart, you can refer to page 40 of this FAA document concerning visual aids. The schematic here describes the standard location of the PAPI relatively to the strip.