Totally depends on the agent, the station, and the flight. Some of the smaller stations that only see smaller planes are the most strict (I recall YQR used to be brutal on QK flights). Certain agents at other stations are super strict (looking at you YYC and SFO). Some seem to only care if they have time, but if there's a delay, it's a free for all. Some will let zone 1 pax get away with murder, others will call out anyone. Its super frustrating.

If you're worried, some tips:
1. Don't hang around the gate before boarding. When they call your zone, then come up. Same goes for check in. Avoid letting them see your bag. If you're traveling with someone, have them hang back with the bags.
2. Take your personal item off the trolley sleeve and either drape it on your shoulder opposite the agent's side, or even better, use a backpack (AC seems to ignore backpacks).
3. Zone 1 is way less likely to get trouble. I've had agents look at my bag, scowl, ask for my BP, look, and then say "thank you" and walk away when in zone 1. Zone 2 sometimes gets less attention too, but don't count on it.
4. If you have a FF tag, make sure its visible on the bag.
5. Don't overstuff - sounds obvious, but people throw in neck pillows and jackets, stuffing them to the point they look ridiculous.


If you're on a CR9, the sizing is actually "legit" - if it doesn't fit in the sizer, it's likely that it actually won't fit on the plane (not like a mainline jet that can easily take a 24"x16"x10"). Its the depth (the 9" requirement) that's usually the problem. If you do get on board, and realize that's a problem and your bag won't fit in the bin, either ask the crew to put it in the closet, or if in Y, put it under the seats. This trick works on the DH4 as well. I recently saw some Flair crew on a QK Q400 commuting, and their bags didn't fit in the bins, but all managed to get them under the seats.