Some inaccurate information being posted here...
First off, TAKE THIS SERIOUSLY!!! The reason is that once the Americans get a hold of your criminal record information, IT WILL NEVER GO AWAY. Doesn't matter what happens here (pardon, expunge, whatever), once the Americans have it they will never delete it from their system. So they key is to deal with it here FIRST before you cross the border and potentially have the info downloaded to the Americans.
Start by going to your local police department and doing a criminal background check. It's not that expensive and they'll tell you right away. If you get a hit, I would avoid going to the US until you can get a pardon. If it's clean then it will only appear in the police department's system in which you were arrested/charged and that information will not show on a CPIC check (which is why you get no hit on a police background check). The hit in the local police department's system shows they arrested/charged you but they can't use that information in a court of law (i.e., it's not a conviction and/or it's been expunged/pardoned).
I'm not that familiar with charges that are expunged when you're a youth but I suspect it doesn't show up as a hit. My understanding is that it's a sealed record that never goes away but CAN be reopened under certain circumstances (i.e., if you're charged with a subsequent crime).
The border guards are smart. They won't ask you if you've been convicted, they'll ask you if you've ever been charged (which means you could have been totally innocent). If you lie they can kick you out and ban you for life from the US (not only is it a criminal offense to lie to a border guard but it's also a criminal offense to NOT disclose that you have a criminal record) so it's best to be truthful. If you were charged but found not guilty then you might be able to get away with saying, "Oh, I thought you meant convicted" but if you were convicted I wouldn't fuck around because they'll make your life miserable.
A pardon will remove a conviction from your record (assuming you can get one) and subsequently it won't show up on a background check. But as I said above, if the US already has your record (i.e., from downloading it when you lied to them at the border crossing), that will never go away. The US doesn't give a shit about Canadian pardons and doesn't recognize them. You can get a US exemption for certain criminal offenses but they only accept certain types (google it, there's a term for it). Anything related to drugs, violence, or sexual offenses won't fly. I don't know about fraud but I wouldn't be surprised if they won't allow that (i.e., fraud involves lying so why would they grant you a waiver if they already know you're a person who lies?). Getting a Canadian pardon helps with getting an American exemption but obviously if you apply for an American exemption then they'll have your record forever.
EDIT: Good point above about getting your CPIC record erased. When you get acquitted or obtain a pardon your lawyer should be writing a letter to the RCMP (yes, I know the OPP administers CPIC) to direct them to delete your file. Not sure if they do it on their own (I think they do but they take their time) but if you send a letter then they will confirm when it's been done.