Ha!
rufio, I'll start with the numerous comparitive studies of deaf children with hearing parents compared to deaf children with deaf parents. Deaf children with deaf parents (deaf of deaf), that is, deaf children who were exposed to language (ASL) from the get-go, are consistently and significantly well ahead on all cognitive and language measurements than those who were not so exposed. It's very pronounced.
Deaf-of-deaf make up about 10% of the deaf population.
In terms of recognizing mental deficiency in specific people, we don't have time machines and alternate universes to test how much smarter someone would have been if they had been exposed to language earlier. But there are patterns, consistent areas of difficulty, etc. I can always tell after interacting with someone for a while if they are deaf-of-deaf, had hearing parents, etc.
This is all very widely known in the deaf community, as well. I can ask, "Did you have deaf parents?" and the person will reply, "No, but I didn't become deaf until I was 8, so I'd already passed the critical language period" -- they know what I'm asking.
First Google result (there are a ton... this is quite easy to research on your own if you are so inclined):
http://www.asha.org/about/publications/leader-online/archives/2002/f021203.htm
Excerpt:
Quote:Not surprisingly, deaf children with deaf parents performed much like hearing children, while the children with hearing parents were significantly delayed in their understanding of a Theory of Mind. The results also show that the deaf children who were delayed in Theory of Mind were equally delayed in both the verbal tasks and the tasks that required minimal language. This means that the children lack some fundamental understanding of how the mind works regardless of whether the task requires language to understand it.