You know you're in Canada when...

[Fearnest]

I put malt vinegar on my fries.

It's just difficult to find malt vinegar in a restaurant in many parts of the US (also depending on the type of restaurant). A fish joint in the northeast is much more likely to have it than a burger joint in West Virginia (or to my experience Alabama, or Missouri, or Kentucky, or... well, you get the idea).

Ditto with mayonaise on your fries - common in some parts, but not in others.

I knew I was back in the northeast about 20 years ago when I could find salt and vinegar chips (crisps) in the store - although those are now common in this part of the country, too.