How can I tell if an assignment has been submitted late?

Any late submissions will show up in the Grades as Blue in the New Gradebook.

Each colour is a default associated with a Gradebook status: late (blue), missing (red), resubmitted (green), dropped (orange), or excused (yellow), as shown below.

The Status option in the New Gradebook allows you to change the default colours in the Gradebook. Colours can be adjusted by selecting a default colour or using a supported hex code.

Colours are a per-user setting. Colours set in your course apply to any of your other courses that use the New Gradebook, however, they do not apply to any other user who may view the Gradebook. Additionally, status names cannot be changed.

In the Speedgrader you can also see if an assignment is submitted late.

Canvas Guide: How do I change the color for a grading status in the New Gradebook?
Canvas Guide: How do I change the status of a submission in the New Gradebook?
For more information please see: The New Canvas Gradebook