There was a time not long ago when even the slightest misstep in a bunker could cost you a penalty stroke. Things like removing stones near your ball or touching the sand with a rake before playing your shot were strictly forbidden. Luckily, when the USGA and R&A got together to modernize the Rules of Golf in 2019, they decided to relax the rules involving bunkers to make them more user-friendly.
One of the nice changes made is that you're now allowed to check beneath the surface of a bunker before playing a shot to make sure that you're not about to slam your beautiful new sand wedge into a massive tree root or an old drain. It's particularly relevant this time of year. The fall in many parts of the country brings weather conditions that cause more wear and tear to courses. And at some places maybe the maintence staff isn't quite as large as it is in the summer, meaning the conditioning of some bunkers could be more ragged than at peak season.
Rule 8.1a and Clarification 8.1a/7 explain that it's OK to probe underneath the surface of a bunker "with a tee or other object to see whether the club might strike a root, rock or obstruction below the surface of the ground when the stroke is made."
The only caveat here is that you can't go poking around if your motive is actually to test the condition of the sand. If you do that, you're in violation of Rule 12.2b(1) and would take either loss of hole in match play or a two-stroke penalty in stroke play.
If you're wondering about what to do if the only thing under a wisp of sand is the bunker's liner, it can be treated as ground under repair—but only if it interferes with your ball or swing. If you're standing on it, you don't get relief.