Depending on your actual requirement you actually have several options.
The first is to work based on an SQL approach by using the "IF Exists ... ELSE ..." approach which you can find details of here:
An alternative would be to run a query prior to your insert command that returns in index of an item if it exists.
Dim cmd As New SqlCommand("Select id from UnitOfMeasurement where unittype='" & txtUnitType.Text & "'", con)
dim response as int = cmd.ExecuteScalar()
IF response > 0 Then
The code here is not 100% as I don't know your column names etc, so it will need a bit of tweaking but is designed to give you an idea.