Help | How to list scheduled messages
You can get a list of the messages you have scheduled using the command /schedule list
Send this command in any channel and a list of your messages will be returned including the time they will be scheduled, the content, the channel and the message ID which can be used to delete specific messages.
The result of the /schedule list command will only be visible to you, but if you have a lot of scheduled messages it’s recommend to send this command to Slackbot so as to not clutter other channels.