This has been one of my favorite subject since the early 1980's.
Everyone's comments have been great and useful!
Let's review some of the possiblites.
Because thermals are equated with heat the first thing most people jump to is infrared. Mike the Strike has pointed out that thermals only have a fraction of a degree difference from surrounding air. TW and David Hurst have also pointed out that air is a very poor radiator of infrared and is invisible to commercial thermal cameras. This is all true.
However I would ask Tom (2G) to please tell us where he heard or read of the military night vision system user seeing thermals over Iraq. That might be a case of intense desert thermals near the ground. Scientific aircraft monitoring has shown that thermals often are cooler than the local air above halfway to cloudbase.
I would also like to ask Jonathan St.Cloud to dig up the reference he had about an Apache trainee. A search for "Apache" in the Soaring mag database does not find anything useful.
Next subject is LIDAR. Lidar is laser radar. It has been shown to be able to detect thermals since the early 1970's. It does this by sensing backscatter from dust and pollen in the air. Very clean air is harder to see. See articles by Eloranta (University of Wisconsin) and many others. Problems are many: the required laser power is large, 100K Watts+ in the pulse, the light-gathering optics are physically large (think 8 inch diameter telescope)as pointed out by someone here, and the cost is huge. You can get a nice little laser that has the required power and is small enough but it costs $75K. Then add the complex electronics that generate and detect picosecond pulses and you see the real-world problems that make this a difficult instrument to cram into a sailplane. It can be done if you have lots of money to spend.
Next possibilty is RADAR. Most of the radar schemes detect bugs.(Raptors most likely see bugs in the air with their superior vision.) No bugs today and your expensive, bulky radar won't work today. A better choice is something called FM-CW radar. This has been shown to work at less power by detection of the moisture differences between the thermal edge and the surrounding air. This scheme may not work in very dry environnments. The amount of power required is in the hundreds of Watts. The antenna is likely to present a very draggy problem.
I won't bore you with other possiblites but there are some that have not been adequetly explored. I'm working on a few. Technology gets us closer every year. It will happen eventually. This will not make soaring boring at all. It's just another useful tool to help you. Probably the first thing to come out will be a short range solution. Many times you can see a cloud, get to it and find nothing. Then you explore all areas under the cloud and on one corner there is lift. A 1000 foot range detector would help greatly in this situation.