Canadians will say "Grade 5" where Americans say "5th grade".
Many Canadians use "eh" as a "filler". There is a joke that this comes from spelling the name of their country: C, eh, N, eh, D, eh
Slight pronunciations of words are the main ways. Attitudes, naming patterns and dress are pretty similar if not exactly the same in most regards. Of course, there are a more distinct regions that have specific cultures and languages/accents in either country…but, if we are talking about the average northern tier Anglophones of the US and the southern tier Anglophones in Canada, they are very similar.
One area that is quite different though is the focus on different sports. Canadians are vastly more interested in hockey. It is not as popular in the US. While the Superbowl is a de facto American holiday and cultural phenomenon. The other main way to tell Americans apart instantly is their measurement systems. They are still on the old imperial system, while the rest of the world (including Canada) has switched to metric. Although, interestingly, there are still some older folks that will use miles/gallons/pounds from time to time.
I always find it funny how people latch onto the differences between the countries and they'll point out how we may saying "zee" and "zed" differently, but this is all very minor. I can go into Canada with my non-regional/Western American accent and be conscious of not saying any "give-away, American" words and I'd be absolutely assumed to be a native-Canadian (or, a First Nation, native English speaker anyway). And vice-versa. You can have someone come down to the states and blend in completely and no one would ever "suspect" that they weren't American.
Bathroom vs. washroom. Americans for the most part say bathroom and Canadians say washroom. This isn't that cut and dry though because some Northerners also say washroom.
Many Canadians pronounce o differently than Americans. Sorry sounds like soary, about sounds like aboat (not aboot like people claim), out sounds like oat, etc.
This was already said, but Americans really know way less about Canada than vice versa. This is one of the few countries where I will say that they know a lot of accurate information about America (other countries often claim they do, but it's usually biased in some way or not complete info).
Canadians talk about the weather, eh? They complain aboot Americans and their funny ways. They sit on the chesterfield and nibble on poutine while Americans sip the horse piss they think is beer on their couch and be disgusted. Newfies can make a US Navy sailor blush but no American can figure out what a Newfie is or where they are from.
Americans have dozens of Interstates they can use to cross the country in any direction on. Canadians get by with a single highway from one coast to the other. And contrary to popular belief, it is not bloody cold 10 months of the year in Canada.
Other ways to spot a Canadian:
Canadian often put vinegar on their French fries
They use British / French spelling, programme, flavour, humour
Their currency is made of Mylar and it's colorful
Canadians from Ontario will be surprised to see beer sold in a convenience store
They drink tea more
Ask the person to name a place to get coffee and donuts.
If they answer "Timmy's", you have a genuine Canadian. "Tim Horton's" may indicate a faker or a Canadian who realizes he's being questioned by an American who will have no idea what "Timmy's" is. (Per capita, there are three times as many Tim Horton's in Canada as there are McD's in the US).
If they answer "Dunkin Donuts", you have an American.