A great place to get an idea would be from the manufacturer’s recommended pressure (Sometimes this is found on the sidewall of the tire, other times it may be molded to the casing - so look closely). The manufacturer may also have a range of recommended pressure (i.e. 40-60 psi) which is why it is only a starting point. Experiment with pressures in the range to find the one that works best for you.
Pump Road Rubber More
Sure, road tires are pumped up to high pressures, but because they are skinny tires, there’s hardly any air inside. That means that when even a little air leaks out (and it will since tires are porous and naturally seep air), the pressure and volume are greatly reduced. To prevent this, check your tire pressure before every ride.