Just went through this with my daughter and, believe me, it's pretty obvious when they're dangerously dehydrated. My girl was completely listless, just a little ragdoll in my arms...which was VERY unusual for any 3 year old. Her eyes were sunken, her face was completely pale and so were her lips. Her mouth was terribly dry, and her teeth had a nasty coat on them that saliva normally takes care of. She had been throwing up non-stop for 36 hours by the time we got to the pediatrician's office, and the doc almost sent her right to the ER. She decided to try anti-nausea medicine first and see if I was able to keep fluids down her at home. Fortunately it worked and she was greatly improved in just a few hours, though she was still VERY sleepy for quite a long time.
So all of this to say that, while mild dehydration might be harder to diagnose (and we probably all suffer from it to some degree), you should definitely be able to tell when it gets bad. I think my biggest mistake was letting her sleep for a six hour stretch at night without pushing fluids (at the time, I thought sleep was more important for her). So just keep an eye on your little man. I'd say it's normal to be sluggish for a while after something so nasty. Taking him into the doc at this point will probably only expose him to more germs, but if you're still worried, giving them a call couldn't hurt.