If you get a customized URL (eg http://steamcommunity.com/id/onkelsam?xml=1 ), you can use the Steam ID converter.
To be pedantic, the custom url would be: http://steamcommunity.com/id/onkelsam
Manually adding "?xml=1" to it will give you the page as an xml output, where you can easily read his steamID at the top:
<steamID64>76561197986270486</steamID64>