Assuming that the values are displayed in a control like a label or a textbox, you can compare the values or the values of the captions. Most likely you have these figures as the Value property of textboxes. This is the default property so you need to be able to reference the controls by their names.
The bigger question is when do you want this message to display? Your options are likely the afterupdate event of the parent control value, assuming it is entered by a user, or the current event of the parent form. Alternatively, if there are line items that you are adding in the sub form, you may want to use the afterupdate event of the subform. Essentially, you need to choose one or more events that are triggered when the record changes or the values to be compared will change.
Assuming you are calling it from a parent form event, the syntax looks like:
If Me.txtBoxValue < Me.subFormControlName.Form.txtCalculated Then
where txtBoxValue is the name of the control on the parent form and subFormControlName is the name of the subform control, not the name of the form embedded as a subform, and txtCalculated is the name of the control containing the calculated value on the subform.
To call it from an event on the subform, the syntax is a bit simpler:
If Me.txtCalculated > Me.Parent.txtBoxValue Then
Edmonton AB Canada