Good morning. The mist through the vents is what you would look for for evidence of the heater core leak, Also if you run your heat you might be able to smell the coolant or a real sweet acidic smell. There isnt a visual check because the dash has to be removed to access the case that the core is in. This is a pretty complicated repair and is also pretty expensive.
Here is detailed instructions for removing the dash and core.
Disconnect the battery ground cable
NOTE: Move the shift lever to the 1 position to ease removal.
Pull to release the three (A) clips and remove the (B) fuse door
If equipped, remove the instrument panel floor duct panel.
Remove the LH and RH cowl side trim panels
Disconnect the brake on/off (BOO) switch electrical connector
Remove the (A) radio ground and the (B) GEM/CTM ground
Disconnect the instrument panel main harness connectors
Disconnect the air bag diagnostic monitor electrical connectors
Disconnect the inertia fuel shutoff switch electrical connector
Disconnect the electronic actuator electrical connector
Disconnect the climate control head vacuum line connector
Remove the instrument panel steering column opening cover reinforcement (04502).
WARNING: ALWAYS WEAR SAFETY GLASSES WHEN REMOVING AND HANDLING AN AIR BAG.
WARNING: CARRY A LIVE AIR BAG WITH THE BAG AND TRIM COVER POINTED AWAY FROM YOUR BODY. AN ACCIDENTAL DEPLOYMENT WILL THEN DEPLOY WITH MINIMAL CHANCE OF INJURY.
WARNING: PLACE A LIVE AIR BAG ON A BENCH OR OTHER SURFACE WITH THE TRIM COVER UP.
Remove the passenger side air bag module (044A74).
Remove the RH assist handle screws
Remove the RH windshield side garnish moulding (03598
Remove the instrument panel bolt below and to the left of the glove compartment
Remove the instrument panel bolts behind the passenger side air bag module opening
Remove the instrument panel bolt on the relay bracket
Remove the instrument panel bolt below and to the left of the cigar lighter and power point
Remove the instrument panel nut on the bottom RH corner of the instrument pane
Remove the instrument panel nut on the bottom LH corner of the instrument panel
Remove the instrument panel bolt on the bottom RH side of the instrument cluster opening
Disconnect the remaining electrical connectors on the steering column
Remove the instrument panel to floor brace bolt.
Remove the instrument panel
Compress the holding tabs and disconnect the heater water hoses at the heater core
Remove the A/C plenum demister adapter (19E662).
Disconnect the vacuum line.
Remove the plenum assembly top.
I hope that this helps you. this is a really hard and indepth job to do, But wil save you a 1000 dollars doing it yourself.