Assuming what you said is correct that

屋主应于21 天内退deposit,如逾期，屋主应退 2x-deposit. How do you demonstrate that 屋主did not于21 天内退deposit. The fact that the couple did not receive the check does not mean they never mailed it out. Set aside the question of burden of proof (whether the landlord sent out the check), you can spend the money on a CA real estate lawyer to find out if what you said above is true and if the landlord did nothing on the deposit or you should just let it go. I would just let it go. Personally I don't think what you cited is true because the point is to get whole, not a windfall (as you may expect).

If what you said is true, it could create a lot of issues. There will be people pretending that they never received the security refund (simply not cashing the check) and request twice the amount after 21 days.

Just let go.