Can you over damp with isolation products? If so - How do you know you've done it?

Yes, certainly. Everything sounds muffled and recessed, no natural reverb....easy to hear over damping.
No, not with isolation devices. You can over-damp with some damping devices or materials but real isolation devices, I.e., mass on spring devices function one-way only as mechanical diodes 🔜. They do two things at once - isolate the component or speaker from seismic type vibration AND dissipate any residual or inherent vibration of the component or speaker. 
Two extreme examples...overdamping may happen when you use squishy sorbothane under all of your gear.  Things will sound lifeless and the bass "fat".  The other extreme might be using Black Diamond Racing cones under everything.  The key is achieving a balance where you are damping vibrations/resonances without going to those extremes.  Herbies's Audio Labs Tenderfeet have been the solution for me.  Not too soft or too in the goldilocks tale...just right!  Trust me, I have a bunch of damping materials that now are relegated to the closet in my listening room.  Experiment if you can and find what sounds like music to you.  Best of Luck on your journey!
If it sounds better when you take the damping objects away, it was over-damped.
@insearchofprat, have you read Shannon Dickson's article "Bad Vibes" on the Stereophile website? You can find the answer to your question (and more) there.
Whether it’s viscoelastic materials, cork, graphite composites, crystals, specialized tapes or mass loading, they all have downsides, some more than others, usually by reflecting energy back into the system. It’s what the excellent Japanese audiophile company, Acoustic Revive, describes as “over dumping” in their literature. A comprehensive program of vibration isolation and use of cones made of very hard materials to allow rapid exit of stored energy is preferable to most damping methods. Isolation itself is effective in damping the component.

geoff kait
machina dynamica
advanced audio concepts
"If so - How do you know you've done it?"

Listen to live music.