If you are super worried, and just want confirmation set up a video camera at an angle where you can see the whole tank and what the eel is doing (probably a bit of a downward shot). Then just put in some food and leave. Review the footage later. You can speed up the playback quite a bit and still know what is going on, just slowing back to normal speed when you think you see eating.
I think that the eel being alive is confirmation enough
Just giving the OP a hard time
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