I believe the below example should get you going:

Function Req_FieldCanChange (FieldName, NewValue)
If FieldName = ''RQ_USER_03'' Then
If Req_Fields.Field(''RQ_USER_03'').Value = '''' Then
Dim ReqNum
ReqNum = NewValue

Set ReqFact = TDConnection.ReqFactory
Set ReqFilter = ReqFact.Filter
ReqFilter.Filter(''RQ_USER_03'') = ReqNum
Set ReqList = ReqFact.NewList(ReqFilter.Text)

If ReqList.Count > 0 Then
Req_FieldCanChange = False
MsgBox ''Req # already exists. Operation aborted.''
Else
Req_FieldCanChange = True
End If

Set ReqList = Nothing
Set ReqFilter = Nothing
Set ReqFact = Nothing
End If
End If
End Function