i know two approaches to this problem.
1. In your table, you could set the REQUIRED property of all entry-required fields to YES. this would also affect if you use it as recordsource for bound forms.
2. Or, everytime you exit the form, you could loop through each controls and check the value to make sure that it is filled.
eg.
Private Sub Form_Unload(Cancel As Integer)
Dim ctl As Control
On Error GoTo Error_Found
For Each ctl In Me.Controls
If TypeOf ctl Is TextBox Or _
TypeOf ctl Is ComboBox Or _
TypeOf ctl Is ListBox Then
If IsNull(ctl) Then
If MsgBox("You are about to exit this form with some fields not complete! Do you to exit?", vbYesNo + vbDefaultButton2) = vbYes Then
DoCmd.Close acForm, Me.Name, acSaveNo
Else
ctl.SetFocus
Cancel = True
Exit For
End If
End If
End If
Next
Exit_Here:
Exit Sub
Error_Found:
DoCmd.GoToControl Screen.PreviousControl.Name
GoTo Exit_Here
End Sub