You can query the system dictionary table with web services if you have the proper rights.

WSDL:

https://myinstance.service-now.com/sys_dictionary.do?WSDL

Then perform a "getRecords" query such as:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
                              xmlns:sys="http://www.service-now.com/sys_dictionary">
   <soapenv:Header/>
   <soapenv:Body>
      <sys:getRecords>
         <name>{TABLE NAME GOES HERE}</name>
      </sys:getRecords>
   </soapenv:Body>
</soapenv:Envelope>

This will give you all of the fields on that table and any information on that field such as data type.

Field names are found in the "element" result. Data types are found in the "internal_type" element.