I use a "L" shaped allen wrench that fits into one of the small holes in the center cap - brace the cap with your other hand and pull...it will pop right off.
Others use a bent coat hanger but my way is much easier, if you have an allen that will fit that is.
... and you should buy a torque wrench
Checking the lug nuts before the 1st run of the day is a fix (safety) habit of mine ...
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I still wave at Boxsters, but they no longer wave back :-(
2002 Boxster S "Violet" (sold but not forgotten)
2009 Carrera 4S "Kelsey" (current ride)
2015 FIAT 500e "Nikki" my commuter car