Once enabled, you will see next to the name of each extension different numbers. For example 1.8.9 - that is the current version of the extension that is installed on your Google Chrome.
Now, comapre it with the number you found for that extension in the Chrome web store. If you number is lower, for example 1.8.9 and in the webstore it is 1.9.0, you are using the latest version of the software.
Although extensions are being updated automatically, if the developer pushed an update recently and you have not used Chrome for a while, it might take some time the automatic update to take place.