You'll know if you've ingested too much by physical symptoms. Primarily, headaches, dizziness and nausea.
These will diminish after you stop ingesting nicotine.
Now, if you drank it, it would be an entirely different process. Potentially very harmful. Potentially fatal.
I can crank through 1ml of juice on an hour if I'm in a chain vaping mood. At stronger nic levels, I'd be feeling very poorly.
I could drink a 12oz can of soda in an hour and not even be chain-sipping. That would be 255 times the amount vaped. Plus, it would be ingested and absorbed at a higher level, than vaping.
In short, you would be very hard pressed to injure yourself by vaping. Even if you tried really hard, you'd likely get unpleasant temporary symptoms.
DW
Typed on my mobile. Please excuse typos.