If the query was successful, the service will return a HTTP code 200.
In addition, if the RESPONSEFORMAT was set to VOTable, the VOTable result document will contain, in the RESOURCE element and before the TABLE element, the following snippet: <INFO name="QUERY_STATUS" value="OK" />.
Example:
<?xml version="1.0" encoding="UTF-8"?>
<VOTABLE ... >
....
<RESOURCE type="results">
<INFO name="QUERY_STATUS" value="OK" />
<TABLE>
...
</TABLE>
</RESOURCE>
</VOTABLE>