To determine whether a string contains an HTML entity, you will be concerned with some information that may not be available by inspecting the string contents: the version of HTML, the character set of the HTML document, the character encoding of the string and possibly even the transfer encoding.
This information might be available from the context in which you obtained the string. For example, if you received it from a web server via HTTP protocol, these attributes might have been specified in the HTTP protocol headers. If not, you may have to make some assumptions, which may or may not be correct. If you make incorrect assumptions, you results may not be satisfactory.
Interpreting HTML documents is complicated by the fact that the character encoding can be specified for some elements in an HTML document, at least with HTML 4.01, so you may have to deal with multiple encodings within the same document. Also, the HTML 4.01 specification has this rather cryptic statement: Character references within comments have no special meaning; they are comment data only. So you might also be concerned when checking your string, to know whether it includes or is from the context of a comment.
Your task may be further complicated by the need to deal with documents which were written to be consistent with some version of some popular browser rather than with any of the HTML or other specifications.
All these difficulties aside, if you assume an HTML version and character encoding then you can search for any of the character entities defined in the HTML specification.
Based on the information you have provided thus far, I would say that the solution provided by wspf is a good start and you should use it until you have some particular case for which it does not work.