Which release (name and date) of the official monero-wallet-cli wallet changed the Monero mnemonic seed standard from 24 to 25 words?
Wordlist
Seems like the wordlist was introduced in 2014 (v0.8.8.5) and didn't change until the 2 minor fixes in 2015 (v0.9.0). That is the same one used at the present.
commit c01069f35253586ac2fbc3d507c7ed3ccdd900ce
Author: Riccardo Spagni <ric@spagni.net>
Date: Wed Mar 4 15:31:24 2015 +0200
fixed English word list issue: 'launchpad' should be 'ourselves'
commit 10e4132e22ccc0f1a41763adee74699ebedb5ffc
Author: Riccardo Spagni <ric@spagni.net>
Date: Wed Mar 4 15:29:20 2015 +0200
fixed English word list issue: 'incline' should have remained 'inline'
commit 010bfcfd18497b5f77cef62d60a56a556dfeadd4
Author: Riccardo Spagni <ric@spagni.net>
Date: Mon Oct 6 08:47:04 2014 +0200
minor English wordlist tweaks
Checksum
The 25th checksum word checksum was also introduced even before the above wordlist.
commit 8f587ba1c8044ed6face883d8f50cd43e27484e3
Author: Oran Juice <oranjuices@hotmail.com>
Date: Sun Sep 28 02:29:25 2014 +0530
CRC Checksum for word seed. Gives a new 25 word seed with checksum if one without checksum is passed. Doxygen comment fix.
Without recovering my wallet from cold storage, is there any way to tell from my looking at my mnemonic seed if it was created with the old 24 word standard or if it was created with the new 25 word standard (and I forgot to record one word)?
Check against the dictionaries, should be either english or english-old. The latter one doesn't use the 25th word AFAIK.