I'm assuming the OP is Canadian visiting the US via air. In that case, they can use the APC kiosks for passport control. Assuming they answer "no" to all the APC questions, then the way to end up needing a secondary interview is either through suspicious activity in the baggage claim area or at the customs checkpoint exiting baggage claim. I think the OP's mission is to get through this whole process answering as few questions as possible - from anybody.

If, for some reason, the OP has to answer more than 1-2 questions about his visit to the US and he has to disclose he his visiting his girlfriend at location X, the #1 concern for border control will be the risk that the Canadian will overstay his welcome in the US. Looking like you're very obviously only taking a 1-week trip (or less) can help manage how deep they tread down that rabbit hole. They would have to dig VERY deep before they ever found out that the resident he was visiting was in fact, overstaying a visa herself.

From those border security shows, they seem to hone in on people travelling with excess baggage. From the point of view of a Border Control Officer, if someone WAS to overstay their welcome visiting a significant other, one thing they would do is travel with a significant chunk of their belongings (multiple, heavy, bags).

So honestly, I'm thinking a clean-cut Canadian OP travelling with a 21" roll-aboard can scoot through an APC and then get in-out of the customs hall without saying practically anything. In some places (DFW is one), if you can manage carry-on only, you go through APC then walk practically straight past the customs agent on the way out the door, even without Global Entry.

More advice for the OP: A lot of data is shared between the US and Canada, if she overstays her visa in the US, there's a chance that may come back to haunt her if she visits Canada someday. Might be worth some digging into.