In the Smart List dialog, select the advanced filter at the very bottom of the filters list.
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Then select "matches" from the dropdown menu and add the following to the input field before saving.

 This means that it will show tasks and projects with no duration estimate whose schedule/do date equals selected date in the main view ("My Day" view).
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Also, don't forget to set  Backburner: Not on Backburner   so that the list filters out items that are on the Backburner. 

You can read more about how to set up advanced Smart List filters here:
​Advanced Smart List Filters and also see Advanced Smart List Examples.

If you want to get a notification when you have an item with no duration estimate you can set up an alert that will be connected to previously created Smart List.

In Smart List Day Alert settings add a new alert and connect it to the Smart List:

Once created, Alert will pop up when there are tasks with no duration estimate:

Upon clicking on the "Assign now" button you will see which items are missing duration estimate:

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