How can I test and increase the water pressure in my apartment?
I don't know if there is an absolute minimum for your area. A pressure gauge can be installed on a kitchen faucet with a adapter to quickly check the pressure.

A few years back a family needed 120 psi for their child's medical needs. I installed a check valve on the main line and then a small booster pump and pressure tank. It was a small pump; I think it was 7gallons per minute. This provided both the pressure and flow they needed. The total cost was close to 1k for the pump tank and install. I also set it up so they could take it with them when they moved.

We had to replace several older valves that leaked due to the high pressure. Their original pressure from the city supply was just under 60. You need to be aware of the possibility of leaks with a pressure increase but you probably won't need such a high pressure.

Also we measured the flow afterwards and had much more than the pump was rated for, probably because the supply pressure to the pump was rated for a shallow well so you may not need as high a flow pump as your total demand.