Here are some BP nodes I found useful for checking if a montage is playing in a specific slot:
IsSlotActive: Return true if this instance has an active montage in the given slot. A UAnimMontage that is playing in the slot and blending out is not determined to be “active”.
GetSlotLocalWeight: Get local weight of any montages this slot node is playing. If this slot is not currently playing a montage, it will return 0.