The easiest way is to remove the presta valve core and inject it through there.
What I would do is
1- remove the core.
2- with the valve at 6 o’clock, use some sort of dipstick to see if there is still sealant in the tire.
3- if it is dry or low, inject sealant through the valve.
4- replace core, and pump up tires.
Assuming the sealent is latex (which it most likely is) you can mix brands. Chances are that if what is in there now is white, it is Stans, if Orange, it is Orange Seal. No guarantee about either, but a reasonable guess.