תנא אלף ושבע מאות קלין וחמורין וגזירות שוות ודקדוקי סופרים נשתכחו בימי אבלו של משה: אמר רבי אבהו אעפ"כ החזירן עתניאל בן קנז מתוך פלפולו
One thousand and seven hundred inferences, and verbal analogies, and minutiae of the scribes were forgotten during the days of mourning for Moses. Rabbi Abbahu says: Even so, Othniel, son of Kenaz, restored them through his sharp mind. (Temura 16)
The same Gemmorah brings the principle of "Not in Heaven" - Rabbis can decide based on their own reasoning, i.g. "invent" new Halochos.
When the Heavenly Halacha is known to be completely forgotten (assuming all Halochos were once known), how do we know if a Halacha was invented anew or restored/revealed to its original state?