How to listen the values existing in the cells then add the reminder in angular?

I have a table to input quarterly data.

In the table, Q1 means quarter one, Jan, Feb and March etc. The user has to input data by the end of the quarter by the current data. Say toady is 3/16/2022 so it is in Q1. Because user can be forgetting to input the data so I want to add the reminder on the top.

My thought is that when page loading, I search the values in cells. If they are null the add the reminder. Also in input event to detect the values in input elements.

But I am not sure if this the best way to do that. Should I use some of listen event to implement it?