How to check heater core leak?

Try putting your air to recirculate and see if you can detect extra humidity on your windshield (that would be the coolant from the heater core hitting your windshield). Try using the heater as the AC might be dehumidifying the air

An auto shop should also be able to do a pressure test for you. But be aware if there is a leak in your heater core and the metal is weak, the pressure will only tear it wider.

Not sure how complicated a Prius dashboard is. I once took weeks to disassemble the dashboard of my 1990 VW Jetta. I did it very slowly and took a lot of notes, labelled ziplocs, and taped them together to make sure I have the screws taped to the correct parts for reassembly later (there were no cellphone cameras back then).

Since it was my daily driver I'd reconnect whatever I needed (ie. instrument cluster, fan / light switches, etc) and prop or let them hang while I drove to the office. Then I'd continue working at home at night. Had to break a few clips attaching the plenum to the firewall to finally get to the heater core and install a $30 heater core from a junk yard.

Putting it back was so much quicker. But I had to disassemble and reassemble that dash a couple of times after that to locate the squeaks that developed after. But after the 1st time, it is very easy to take off again.

 

Did you bleed air out of the CHRS system? If not, do that first before you become concerned about possible leaks.

It will not be very easy to get to the cabin heater core, the dashboard would have to be substantially disassembled for access.

 

If you're not getting coolant into the passenger cabin (liquid, vapor or odor), you probably don't have a heater core leak. If it were colder out, you'd be able to see coolant on the windshield in defrost mode. You should at least smell it.

What methods are you using to bleed the air from the engine coolant system? Are you using the upper bleeder valve on the radiator as well as running the CHRS pump? This is a very difficult system to bleed. The dealer mechanics often don't get it done right either.