The short answer is that you usually can’t, not when you’re starting to do the experimental section itself. Sometimes there’s something noticeably off about a section, but not usually.
The longer answer is that there’s not really an advantage to identifying the experimental section while you’re taking the test, anyway, so there’s no point thinking about it.
The smartest approach to the test is to treat every section, and every question, as though it counts toward your score. Sometimes people try to guess which section is the experimental one and then coast through it, hoping to conserve their energy for the rest of the test, but this is a really bad idea–if you guess wrong about which section doesn’t count, you could seriously damage your score, and even if you guess right, conserving your mental energy isn’t likely to make much of a positive impact on your score.
So don’t worry about which section is experimental while you’re taking the test. Just worry about answering every question correctly.
After you take the test, you might want to try to figure out which section was the experimental one so you can try to see if you did well on the test overall–people usually do this by comparing the sections they had on their test forms to see which section was unique. Of course, you’re free to do this if you feel like it, but I strongly advise you to adopt a principle for the LSAT that smart law students have adopted since time immemorial: don’t post-mortem a test 🙂