How does Saral know a return has been filed?

Open this screen in Saral/accounts/hmrc-dues?tab=returns

Either Upload filed returns on HMRC Dues — VAT returns, CIS300s and CT600s already filed with HMRC, one file, many or a whole folder; each is read, stored under its company and period and marked as filed — or record the filing in the period's drawer on Returns (step 2 File: the date, the figures and the return document). Update never records a filing.

Not what you needed? Inside Saral, ask Kuku (Alt+K) — if it cannot answer, “Ask a human” sends the question to Saral support with the chat attached.