With engine hot squeeze the upper rad hose.
If the hose is firm due to pressure inside, the system is holding pressure.
If not firm then it's leaking.
Can do this right after shutoff or with it running.

Of course you should check check for visible leaks and a drop in reservoir level. But the squeeze test can be done as soon as you get it fully warmed up.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.