Thermal flight is when you are already airborne and find a column of rising air and circle in it. Thermals happen as the ground warms and the warm air rises

If you have ever seen a hawk, buzzard, etc... circling and gaining altitude, most likely they are riding a thermal air mass.

You still need some sort of way to get up in the air. A powered glider is just the thing. You hand launch it, use the motor to gain altitude, kill the motor and glide around. I see guys at our filed use the motor to get a lot of altitude, kill the motor and go where the hawks are circling and fly with them, it's also the coolest thing when the hawks come over to see what kind of bird that is (meaning the glider) and form up with it because the pilot found the thermal

You will know when you are in one as you will see the plane gain altitude and in a powerful thermal, you see that pretty quickly. The trick is to circle around inside the thermal. You can tell where the thermals outside bounderies are as you leave the edge, the plane starts to lose altitude. You then circle back 180 degrees to get back into it. Pretty soon you are making a circle. Watching the wingtipswill also tell you alot about the edge of the thermal

I may not be totally acurate here but it's a good start. I myself have never flown a glider but watching guys at our filed and know full scale gliders, I have picked up some tips on doing it. I have even considered the same Goldberg plane you are as I like Goldberg ARF's and own 2 of them myslef (An Ultimate Biplane and the Sukhoi SU26)