Dispensers (and Droppers) have a `triggered` boolean blockstate flag, which presumably is set whenever the dispenser is receiving redstone power.

However, actual behavior is that the flag will not update unless the inventory is opened, at which point it does update (this, powering and unpowering the block will not change the state unless the inventory is accessed). This is most notable when using a multipart model for a Dispenser, but can be detected with the usual debug screen.