Required fields (marked as such in the List Settings) are automatically identified and marked in the form designer (IsRequired property).
You can mark any field as required by raising the "Required" flag in the control properties window.
Inside the designer, in the "Options" tab, there are 2 options for required fields:
Required Field Border and Mark Required Fields.
The first property defines the border color for required fields and the second one sets if the form should apply the required field color.
NOTE : The cells containing required fields are automatically given the predefined "pf-required-field" class which you can override by adding a style element in the Scripts > Includes section of the designer.
For example :
<style>
.pf-required-field {border:solid 2px #A00;}
</style>
The above script will change the required controls to something like the following :