I am teaching a SAT prep class at our homeschool co-op this year and have been learning quite a bit in the process. I don't know that I would say there's a guessing penalty on the SAT.
First of all, the multiple choice questions give you 5 possible answers for each question.
You do get 1 point for every correct answer, 0 points for a blank answer and -1/4 point for an incorrect answer. By random chance, you would get 0 points if you guessed on every single answer.
So, if you just looked at the odds, you should be able to randomly guess the correct answer 1 time for every 5 questions you answer (20% of the time).
Let's use the following example. Say you get a $1 bill for every correct answer (1 point) and you lose a quarter for every incorrect answer (-1/4 point.) If you guessed correctly on Question #1 you would receive $1. If you guessed incorrectly on Questions 2-5, you would lose a quarter each time for a total loss of $1. Therefore you would be at $0 (or 0 points) by randomly guessing on all 5 questions.
We know that real life doesn't always work like statistics, so in any individual test you might be points ahead or points behind by randomly guessing.
However, the important part to take away from this is that whenever you can eliminate even one (or hopefully more) of the incorrect answers, you increase your chances of getting points even if you have to guess.
Respectfully, S Dean
Anyone can take an AP test, even if they have not taken an AP course. The tests are really hard, really long, and the student needs to be prepared.