https health monitor configured with only send strings but no receive strings - what it will check ?

Per F5 documentation:

"Content check monitors are designed to determine whether a service is available and whether the server is serving the appropriate content."

Also in the same article, "You can also use the default null Receive String value [""] [in a content check monitor]. In this case, any content retrieved is considered a match." (Emphasis mine.)

In other words, without a receive string, a content check monitor satisfies only its first purpose - "to determine whether a service is available." To satisfy the second purpose of a content check monitor - "to determine if the server is serving the appropriate content" - you must include a Receive String with the appropriate content. This is by design, as F5 cannot possibly know what constitutes appropriate content for your application. We provide the monitor shell; you customize it for your application delivery needs.