Download Google Chrome, open your site, hit Ctrl+Shift+J, click Resources, look for loooooong lines and see what they are. Everything to the right of blue line is mostly safe (async).

Also open your HTML source and search for src="http, look if any of those are <script tags. Async loading scripts don't use src="http part.

Update: http://whiteposts.com/not-async

If you're using Firefox, the Firebug extension's Net panel (when active) shows the Gantt chart of downloads.

Safari lets you profile a web site too. It even has nice little graphs to show you what is so slow.

http://www.webpagetest.org/ - is very nice