You can buy a pressure gauge at the local hardware stores for a couple bucks, you can then plumb that into a line/hose/faucet to check your pressure.
Anything less than 60psi and you'll want to look into a booster pump.
6stages = more filters = more $$ to change those filters... Not that your thought process is wrong, but unless your TDS is excessive it won't give you much benfit over a lower stage #, as the end result is still 0tds.
You can buy a handheld TDS Meter for <$20, either online or some LFS carry them. You may want to check your TDS before pulling the trigger on your RO/DI