It does it automatically, but if you are curious about a certain app, you can check it. Just view the app in the marketplace on the phone, and then hit back, and then click on the app again. If there is an update, "Update" will appear at the bottom.
TumbleBee (Tumblr client) got updated few days ago
The name is changed to BumbleBee, earning it a higher position in the alphabetically sorted application list.