A memory can begin clearly and still leave a reader unsure when the present scene resumes. If your draft moves into the past, inspect both ends of the passage. The return deserves as much attention as the entrance.
Find the present action you interrupted
Write a plain sentence describing what the character was doing before the memory began. Perhaps they were waiting outside a locked room. Keep that action nearby while reading the flashback: what changes in our understanding of the wait?
If the memory supplies background but has no bearing on the current moment, consider shortening it or moving it elsewhere. Relevance does not require an explicit explanation, but you should know what the passage contributes.
Reconnect through a concrete detail
At the return, use an action, sound, or object that belongs to the present scene. A key turning in the lock may do more work than a vague phrase about coming back to reality. Check that the next sentence continues from this renewed position.
Ask a reader to mark where they think the flashback ends. If their mark differs from yours, inspect the transition before adding more background. The aim is not to announce every time shift mechanically; it is to give the reader enough orientation to experience the shift without stopping to reconstruct it.