Assuming the pool member responds to the send string with an HTTP 200 status, using a receive string of "200 OK" should work fine:
curl -I www.yahoo.com
HTTP/1.1
200 OK
Date: Tue, 20 Jan 2009 15:14:39 GMT
P3P: policyref="http://info.yahoo.com/w3c/p3p.xml", CP="CAO DSP COR CUR ADM DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE LOC GOV"
Cache-Control: private
Vary: User-Agent
X-XRDS-Location: http://open.login.yahooapis.com/openid20/www.yahoo.com/xrds
Last-Modified: Tue, 20 Jan 2009 15:02:20 GMT
Accept-Ranges: bytes
Content-Length: 9562
Connection: close
Content-Type: text/html; charset=utf-8
Aaron