OK, so I'm trying to basically add an indicator of any kind in one field based on another field having data entered v no data entered. Ideally would like to display a red flag, or some kind of icon, but if its too complicated a simple change of color to the field (based on another) I'll make due with.
You can make use of Client Side Rendering (CSR) to add an indicator based on values.
In the example shown here, icon is shown based on Yes/No field. So you can achieve something similar to this.
Or you can change the color of the column value conditionally as shown in another example.
You can go through many such examples of CSR here. You can implement the CSR in List Views as well as on List Forms.