If you have your mail held or forwarded when you travel, the post office has a really useful new service called "Informed Delivery". You can sign up online, then they will send you an email showing an image of all letter sized mail that is scheduled to be delivered that day. You receive this whether your mail is actually being delivered or it is being held.
If your mail is being forwarded, you can sign up for a second address (the one it's being forwarded to) and you will see what's being forwarded to the second address.