"I did say “if it has been identified for development”"
Yes, of course, but in this context we probably need to qualify by saying if it has been identified for development by the Local Authority.
It may have been identified as suitable for development by a developer or land agent, outside of the LPA's knowledge.
As you say, the final sale price will be subject to Planning, including the costs of obtaining the permission (which will be deducted from a 'gross' land value that has been agreed at the outset), but that won't concern the OP, I don't suppose!
Parish Councils usually can't find their bum with both hands, in my experience, so the Planning department of the Local Authority is the place to start if you do want to make such enquiries, but don't make assumptions that the site isn't being brought forward if you draw a blank.
"At the very least there should be evidence of housing need and no developer can build social housing because they feel like it. (Well not yet anyway!)"
There are, however, good recent appeal decisions that basically say (to paraphrase) that we've got a critical, national housing shortage and LPA's should not resist sustainable housing development (affordable or otherwise), just because they've met their numbers on local need. If the Local Plan is out of date, this is doubly so.