How can I find out if there are still spaces available in the long term garage

Never saw it full when I used it. Knock on wood

You can view how many spaces are available in each level of the domestic long term parking garage here: https://www.sfoconnect.com/parking-availability

With the new addition, I've never seen it completely full. No way to tell until you get there as far as I know.

I believe They have electronic signs that state how many spaces are available.