meihuak I don't think there's any easy way from the forum UI.
If you don't remember the discussion title, you'll have to look through every recent discussion to see if there are any deleted posts near the end.
I think you can find soft-deleted posts using the REST API, but there's no UI for it.
You can easily find soft-deleted discussions using the is:hidden search gambit but that's probably not helpful for this situation.
If you have my Audit Log (free or pro) enabled, you can find the post ID including a direct link to the post inside of the audit window for the (non)approval event.
If you don't want Flarum to redirect you to the next flag, I think you can instead open each link from the flags dropdown into a new tab. Then Flarum shouldn't try to automatically go to the next flag.