Need advice on reliable way to identify IE as the browser

Have a CGI app that builds a page with some Javascript - it can try to use an ActiveX object, but, of course, that is invalid in Firefox - or at least I will treat it as such.

I've looked at some ways to know that the browser is IE - UserAgent I think is one but things like Opera can make themselves look like IE for part of it.

The use of the ActiveX object involves a javascript:void that calls a function. That function tries to execute a function in the ActiveX object. I have tried a try-catch around the javascript:void but am seeing - doh - that there will never be an error at that point that I can respond to - or is there?

In firefox, the call to the ActiveX function does generate an error, but it's buried in the javascript:void.

So what are my options? Back to useragent or whatever it's called? - have to check google again. Or is there a way to check some error back where I use the javascript:void?

Or something else!!

Thanks
Vern