How can I determine the STC (Sound Transmission Class) rating for my condo walls and ceiling?
I lived in Southern Calif. in the '70s, I'm pretty sure there was no STC standard for multi-family residences at the time. The only requirement was for certain party walls to be fire rated, and not necessarily every party wall. There are publications available that show the STC ratings for various constructed assemblies. You can find an assembly that is similar and get some idea of the rating for your assembly. However, you cannot claim your assembly achieves a certain STC rating unless it is built EXACTLY as the published example. If there is any variation, you cannot claim any STC value at all without having it tested.

Still, reviewing the various assemblies will give you an idea of what needs to be done to achieve a reasonable rating. BTW, much like water proofing, there is no such thing as soundproof, only degrees of sound resistance. Note it is called a Sound Transmission Coefficient. The implication being some sort of sound will be transmitted in all cases.

Perhaps the biggest issue with applying sound treatment to an existing building is your options for eliminating flanking paths is very limited. Even if you build a very effective sound wall, unless you address how the sound can travel through the adjoining floor, ceiling, and sidewalls, it may have only minimal effect.

That said, I applaud your consideration of your neighbors. Doing something is certainly better than doing nothing. But do not think you can blast your audio system with impunity just because your sound treated the one common wall. Effective sound treatment requires addressing ALL the related construction adjoining the wall in common.