Not often, as far as I can tell. Especially at the land border. Presumably they're testing someone, but it's not something we're hearing much about.
Your proof of vaccination gets uploaded to ArriveCAN in advance. You no longer need a quarantine plan or pre-arrival tests, but you still need to enter your trip in ArriveCAN. Make sure you do this.
Also, be prepared to wait awhile. When we crossed in March it wasn't busy, and the lines were relatively short, but many cars were being held for easily 5-10 minutes or more (and then invariably sent to secondary.) The trouble is that it's impossible to choose a line, because all it takes is one person in front of you who doesn't have their paperwork in order to hold up the line. When we finally got to the front it only took us a couple of minutes to clear, but it took awhile to get there.
Hopefully it will be better now that there are fewer requirements in place. There's just less that can go wrong.