There are two technologies that can see thermals: radar and lidar. When Doppler lidars or radars ("wind profilers") look straight up they can see the vertical motion in thermals. Lidar gives much better sensitivity and resolution but it cannot see inside clouds so it can only be used for clear air. The wind profiling radars (wavelengths about 30cm) can see both in and out of cloud, but the signal is weaker, so one needs a fair bit of sensitivity or power to resolve an individual thermal.
Our profiler in AES (Atmospheric Environment Service of Canada) usually needs to collect data for 30 seconds or more to get a good signal, so we probably miss individual small thermals. Nonetheless one does definitely see patterns that are probably thermals. Ours is a commercial system. Research radars with more power and sensitivity have seen more detail. In addition, some wind profilers are equipped with something called RASS (Radio Acoustic Sounding System) which can measure temperature. A few years ago our systems was used by some McGill researchers to do heat flux estimates by correlating motion and temperature variations.
Leaving remote sensing technology aside, glider pilots look for birds soaring in thermals. It has been said that flying insects get caught in thermals and can give a return on some sensitive radars.