My 48 cm cervelo RS has a 73 degree STA, and I love it. I have long femurs and need a 73 degree or less STA to get set up properly. Saddle matters too. I use a selle italia diva gel flow which has nice long rails.

While you could try to measure it with a protracter, I think it will be hard to get an accurate measurement. Perhaps if it was immobolized in an instrument like you might find in a frame shop. But maybe bringing your saddle seatpost with you, dropping a plumb line and measuring where it falls relative to your bottom bracket would work (but make sure bike is perfectly level, perhaps put in a stand and then be sure both wheels are same distance to ground.)