Checking if a number is blocked on your phone will vary by phone, each may use its own variation of software, and you don't mention which phone you're using, so you may need to consult your owner’s manual.
However since Apple's iPhone is the most popular smartphone in the U.S these days, we'll take a look at how to figure out who's blocked under iOS 7 and iOS 8.
It's pretty easy, and these tips should solve your problem in just a few seconds: Open the Settings app and tap on Phone, and then Blocked.
That's basically it, you'll be presented with a list of any contacts which you have blocked.
You can block another contact by tapping the Add New... button, and should you decide to unblock one of them simply, swipe right to left on the contact and an Unblock button will appear, tap that and the contact is no longer blocked.
If you're using an HTC One, the steps are fairly similar to that of an iPhone: go to Applications > Contacts > Menu, and tap Manage Contacts and you'll find the Blocked Contacts option.
For some other Android based phones check out WCCF's Android Tips and Tricks: How to Block Phone Calls on Android – No Apps Required!.
In "that's a lot of phones!" news, CNET is reporting that iPhone 6 preorders in China have soared past 20,000,000.