Determine If A HTTP Request Is A Soap Request On HttpApplication.AuthenticateRequest

I there a way to know if a request is a soap request on AuthenticateRequest event for HttpApplication? Checking ServerVariables["HTTP_SOAPACTION"] seems to not be working all the time.

public void Init(HttpApplication context) {
context.AuthenticateRequest += new EventHandler(AuthenticateRequest);
}
protected void AuthenticateRequest(object sender, EventArgs e) {
app = sender as HttpApplication;
if (app.Request.ServerVariables["HTTP_SOAPACTION"] != null) {
// a few requests do not enter here, but my webservice class still executing
// ...
}
}
I have disabled HTTP POST and HTTP GET for webservices in my web.config file.
<webServices>
<protocols>
<remove name="HttpGet" />
<remove name="HttpPost" />
<add name="AnyHttpSoap" />
</protocols>
</webServices>
Looking at ContentType for soap+xml only partially solves my problem. For example,
Cache-Control: no-cache
Connection: Keep-Alive
Content-Length: 1131
Content-Type: text/xml
Accept: */*
Accept-Encoding: gzip, deflate
Accept-Language: ro
Host: localhost
mymethod: urn:[URL]

Some clients instead of having the standard header SOAPAction: [URL], have someting like in example above. "mymethod" represents the method in my web service class with [WebMethod] attribute on it and [URL] is the namespace of the webservice. Still the service works perfectly normal. The consumers use different frameworks (NuSOAP from PHP, .NET, Java, etc).