Thank you wvmarle and Grumpy_Mike for your ideas!
I think it is correct that magnetic organs are not an option since they would stick to the tweezers. I also considered optical detection but I do not like the idea a lot, because it seems like a lot of work and material (for each slot) and I'm not convinced about getting it reliably to work.
I do like the idea of capacitive sensing. I did a quick research and apparently a simple piece of aluminium foil below each slot could do the trick. However I'm also here skeptical about the reliability of this detection idea.
Has anybody already experience with this kind of capacitive sensors? How reliable and at which distance do they work? What are the important parameters here? Geometry and material of the measurement electrode and the detected objects? What works best then?
Again I'm very grateful if somebody can share his knowledge and experience with me. Either way I will play around a little bit and report back. And of course I'm still open for other suggestions if anybody has an even smarter idea.