J.F.
I just bought one from Walmart for about $30 about a month ago (if you buy online it is in the $50-60 range). It is called earcheck and does just what you said. It comes back with a reading that says if there is fluid. It gives a rating from 1-5. If you get a 1 or 2 it is ok. If you get 3-5 it says go see a docter. My son has gone through numerous ear infections and so we used it about every day to check his status. Sure enough, the day it went to 3 I took him into the doc and he had an infection.
The only problem we have seen with it is if ear wax gets on the ear drum then we get an x and it says it can't get an accurate reading. We had been getting the x for a couple of weeks and then went in to my son's 3 yr checkup and the doc said she could see some wax covering the ear drum and she said that was probably why the device wasn't working.
By the way if your child is getting tons of ear infections you may want to try having your child go to a chiropractor and/or switch to rice milk if old enough. Like I said my son has had chronic ear infections and has gone through 2 sets of tubes. We were heading down the path for a 3rd set and I really didn't want him to go through that again so I took him to a chiropractor that specializes in children. This helped considerably but did not eliminate the infections. However, once we switched to rice milk instead of cow's milk my son's congestion disappeared and haven't had 1 infection since.
If you want to find out more about the chiropractor just email me. Goodluck.