Look around the outside edge of the outer tire - try to find one of the lead wheel balancing weights. There is usually a small gap between the white steel wheel and the shiny aluminum wheel cover at that point. Using a rather large flathead screwdriver, insert it into that gap and pry the wheel cover outward.
Work your way around the wheel until you can get your fingers in enough to pull outward. Usually at about the halfway point the wheel cover will pop off.
Look carefully through the holes in the white wheel... you will find the valve stem for the inner tire. BUT... without using a valve extension, it may be difficult/impossible to get an inflation chuck onto the valve. Another option is to use a flexible rubber chuck.
Be careful with tire valve extensions. Although many people use them, they are probably the largest source of leaks. Also, if the longer ones are not mounted properly, they can (and do) rub against the wheel and eventually fail. Do your research then decide. Big rig trucks use them all the time... BUT they don't use the cheap ones, and are usually PROFESSIONALLY installed. NOTE: NEVER use valve extensions on rubber valve stems!!
If your budget allows, consider a TPMS (tire pressure monitor) system. It's somewhat of an upfront expense, but you'll find yourself taking off those rear wheel covers much less frequently... AND peace of mind while on the road!