A TDM file is an XML file where it's first bytes are the standard xml intro:
<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
So detecting a TDM file by the first three bytes is not possible, becausse the same XML preamble can be contained in any XML file.
To detect wether this is a TDM file or not, at least the model section needs to contain the TDM data model:
<usi:model modelName="National Instruments USI generated meta file" modelVersion="1.0">
<usi:include nsUri="http://www.ni.com/DataModels/USI/TDM/1_0"/>
</usi:model>
You may use a XML tool to extract that information....
