da página man check_http "-s, --string = STRING Cadeia de caracteres a ser esperada no conteúdo"
você tentou com "-s"?
$ ./check_http -I localhost -p 80 -s True -u /test.html
HTTP CRITICAL: HTTP/1.1 200 OK - string 'True' not found on 'http://localhost:80/test.html' - 419 bytes in 0.001 second response time |time=0.000667s;;;0.000000 size=419B;;;0
$ ./check_http -I localhost -p 80 -s true -u /test.html
HTTP OK: HTTP/1.1 200 OK - 419 bytes in 0.001 second response time |time=0.000776s;;;0.000000 size=419B;;;0
$ cat /var/www/html/test.html
<td class="row-left" bgcolor="#FFFFFF" rowspan="2"><small>Applicationname</small></td>
<td class="row-center" bgcolor="#FFFFFF" rowspan="2"><small>true</small></td>