They are far too tiny to probe, you would probably severely damage them. Baby garters are very, very easy to sex by popping. In fact, I am at the point now where even before I pop them, I can visually tell the males from the females by looking at the tails. The males have very prominent "parts" and they pop very readily and with little pressure. This is of course, assuming that you know how to pop them and you are able to do it safely!