It's not an audible vario but the airplane gives you all kinds of clues as to what's going on.

Get the plane trimmed for minimum sink and then watch the airplane. If a wing is tipped up, there is a parcel of rising air under the up-going wing. If the "tip" is a good prolonged one (a second or so) it's most likely a well formed thermal instead of just a random disturbance. Turn about 120 degrees toward the rising wing and see if the nose dips slightly or the plane gain speed before rising. You're in the thermal, begin circling. If going up, tighten the turn, if climb slows down, widen the turn until it increases climb rate. Take wind drift into account.

If you're flying straight and level and the nose drops or the plane speeds up, wait for the plane to start to climb and then begin circling, chances are you will core it right away.

Thermalling is a skill that you have to develope but thermalling a 2 meter model sailplane is much much easier that thermalling a hangglider or real sailplane because the size of the model. Where a hanglider/ real sailplane may get to stick a wing into a thermal, a model sailplane will be fully engulfed in upwards moving air. I've had my sailplane heeled over 45-60 degrees and still going up... It's great fun. But it takes practice, practice, practice.

Don