The BC Hydro smartmeters can easily tell if a unit is vacant. There would have to be an actual law on the books to get that data however. And no, you're not going to cheat those.
yes I will. A device like a NEST thermostat or ecobee can be programmed to randomly turn the AC/heat on a few times per month. A little box that randomly turns things on or off is trivial to make and I could prototype something in a weekend. /r/homeautomation does things like this all the time with existing products. Given that this is a billion dollar industry, such products will spring up immediately. Spending ~$50ish dollars per month to drive the electricity is a small price to pay compared to any kind of percentage based tax.
It's also easy to just ask ICBC what cars are registered to a property, and look for units that have no car as a starting point.
how have people go door to door? Camp out for weeks/months in from of a person's home to catch them?
And the easiest thing to do is bounce an addressed mailer off Canada Post (eg like a Census) with a due date, and those that don't respond go on the "suspected vacant property" list.
There are already property management companies that handle maintenance of the outside and inside of the property. All they would have to do is add a mail collection service. Again, it's just a small cost that cuts into profits a bit, but the profits are so great it's still worth it. At the extreme, you could even just hire someone to stay at your place for a bit. Let's say the laws say 90 consecutive days with no occupant, all you have to do is have one person who goes to a new house every day and stays the night.
Plus there is always just "rat out your neighbors".
this is what it always comes down do, and I absolutely abhor the idea of having everyone on the lookout waiting to rat on their neighbours. Having government employees staking out people's homes just feels even worse. The housing problem is bad, I don't think making a society where we constantly try to report each other is the solution.
I want to deal with this problem as much as anyone else, but I don't want knee jerk reactionary laws that end up making society worse for regular people while just getting loopholed on day 1 by the people it's actually meant to police.