One way to know is if your condenser coils freeze when the heat is on or if the evaporator coils freeze when the cooling is on. Another way to know is if you hear or see any liquid dripping. The liquid is just water, but it is still a concern. The last way to tell if the freon is low is if your mini split isn't working well. If the mini split is not as efficient as it should be, it may need more freon.