Overall the easiest place to start is a forum for your specific vehicle. You may get lucky with predefined services or someone else having done the reverse engineering for you.
If you don't get lucky, it depends on the manufacturer. In the case of European manufacturers you are best off if you can acquire the ASAM ODX material for your vehicle (sadly there is not an easy "clear market" way to do this - reverse engineering the manufacturer's diagnostic tool is often easiest). This will define in a very complete manner the available services on your vehicle. Some aspects will still be proprietary, usually specified as DLL files referenced in the ODX (for example, the Seed/Key authentication algorithm for flashing VW vehicles). These you will have to find references to on forums or reverse engineer yourself.