The gemara says that if dry measurements aren't able to give kedushas ha'guf, then the lechem ha'panim couldn't possibly become chometz after it was already kedushas ha'guf. Tosafos asks, perhaps after being baked in the oven to the point of נקרמו פניה - browning on the face of it, the oven can give it kedusha, and if it is then removed it is chometz? Tosafos says that we derive from here that the shiur of נקרמו פניה is considered fully baked and can no longer turn to chometz. Tosafos says that this would be a good indication for matza on pesach being fully baked (presumably this shiur only makes sense at a temperature that is suitable for baking it properly on the inside, but not if the matzah is thick and the oven is very hot so that it will just burn on the outside).
The Shulchan Aruch (461:3) that the shiur to determine that matzah has been fully baked is that אין חוטין נמשכין ממנה - no strings pull when it is broken. The Rama warns that because of this, it shouldn't be removed from the oven early even if it will immediately be returned. The M.B. explains that since it has heated up, if it is removed while still having חוטין נמשכין, it can turn into chametz very quickly. However, if it was removed and immediately returned to the oven, it can be used on pesach but not for the mitzvah of matzah on the first night (m.b. and sha'ar hatziyun 26) - we assume that it is not chometz but isn't considered to be שמורה - it isn't watched from being chometz.
The Sharei Teshuva (also m.b.) cites the nodeh b'yehuda (tinyana 80) who explains that since נקרמו פניה and אין חוטין נמשכין are the same shiur, if one is unsure about one they can use the other to determine. The N.B. says that if the matzah has already cooled זמן מה - for some time, so that חוטין נמשכין wouldn't be a good test - כי בדיקה זו לא שייכה אחר שכבר שהתה זמן מה אחר שמוציאה מן התנור ונצטננה, then one can check for נקרמו פניה to be matir it. However, if it fails both tests, the biur halacha explains that after a shiur mil it is considered definite chometz, not like the magen avrohom who is lenient bidieved.
Regarding the test of חוטין נמשכין, the Mishna Berura (14) says that it only works while warm, not after it cools, citing the Nodeh B'yehuda as his source. The Chazon Ish explains that the N.B. implies that if it is determined to be אין חוטין נמשכים right after cooling it is fine. The N.B. only says that it can't be checked much afterward, but simply cooling doesn't prevent the test of חוטין נמשכין.