Between ground and the oscillator output but be aware that the probes, wiring and meter will probably pull the frequency to some extent. When you connect them, they will appear to the oscillator to be inductors and capacitors connected across the output and the frequency could shift significantly, it could even stop oscillating altogether.
Real RF probes are very low capacitance and high impedance, often they have amplifiers inside the probes to isolate the measuring equipment from the point they are connected to.
Brian.