How to determine when a Mac is in Power Nap state?
There is the simple answer and the clever answer.

The simple answer is listen for the external drive spinning. If it's sleeping, you will not be interrupting data transfer (but you will be yanking an open filesystem from a sleeping computer but that's not a new issue introduced by Power Nap).

The more clever answer is to observe other things that may be happening like network traffic by sniffing the traffic and seeing if the machine is sharing files or available to ssh into if you have sharing turned on. This takes more work and an iOS device or another computer and certainly won't be faster than just opening the lid of the Mac and knowing it's not napping anymore.

In the end - it's probably worth re-training your workflow to just open the lid and eject things properly. Since Power Nap is only supported on Mac that have Apple supplied integral SSD and ridiculously fast wake times, the delay to perform the eject cleanly is less than 10 seconds excluding however long your external drive takes to wake.