I would add antifreeze
(straight or 50/50 depending on what is in there now), start it up, put it in gear
and run it 1000 rpm
for 10 minutes. That will be enough to open the thermostat, circulate and mix the new stuff.
If you want to be absolutely, absolutely sure, I think there is an antifreeze
drain cock on the back port side of the engine
. Drain a sample and test, either in your freezer
or with an antifreeze tester.