You will find the date of the last NTM that was used to compile the almanac inside the book. This is usualy somewhere near the front.
Most almanacs are only correct up to about week 30 of the previous year NTM's due to the time it takes to assemble and then print ready for sale at Southampton Boat Show. So it's most important that you get the corrections from the relevant web site or publisher.
depending on the submission they could add it to the updates they produce for each edition. For the 2007 eadition I received at least 4 update notification by email.