Checking for the existence of a pushState()
method on the global history object should be sufficient.
function supports_history_api() {
return !!(window.history && history.pushState);
}
For more general HTML 5 feature detection I'd look at Modernizer
http://diveintohtml5.info/detect.html#modernizr
This will insulate your code from the messy specifics of each test, making the code more readable and less error prone. With the Modernizer script on your page you'd just do:
if (Modernizr.history) {
// history management works!
} else {
// no history support :(
// fall back to a scripted solution like History.js
}