When I need an RF sampler
Unless you have an atypical oscilloscope or an RF transformer in there, the low voltage rails of the pi mean the amplitude of the pi's output is probably well within the range of what the scope can tolerate with appropriate settings, and if not a 10x or 100x probe should probably solve that.

That leaves the question of how best to load the pi "transmitter" - probably you want to load it with a non-inductive resistor of some sort, as the scope input itself will be too high in impedance.

Another option would be an RF attenuator - essentially a specialized voltage divider, which will itself help present something of a load (though not quite of the intended impedance, unless terminated in the designed impedance).

Finally there is the question of what you hope to see.  Depending on the operating frequency and scope, the fundamental component might be within the scope's bandwidth, but time domain instruments such as scopes are incapable of telling you the important things about an RF signal, such as harmonic content, frequency, or noise to anywhere remotely close to the degree which matters.  Modern digital scopes may have a frequency domain FFT mode, but their digitizer is typically only 8-bits - far from the dynamic range needed for useful RF measurements.

If the signal is not within the bandwidth of the scope you could build a diode power detector - basically a "crystal radio" used to measure the envelope of a directly connected source/transmitter.  A scope's high impedance input is a good match for these, which would otherwise need a high-impedance voltmeter.

Past versions of the ARRL handbook used to include plans for a heterodyne down converter for looking at HF / low VHF signals with a low bandwidth oscilloscope.  Today some people use DVB TV tuner dongles as crude widely tunable software defined receivers - these have some serious limitations in frequency and instantaneous dynamic range (8-bit ADC) and who knows what in terms of flatness over frequency, but can be useful when their limitations are kept in mind.

Various other types of RF measurements require sampler circuits for reasons beyond just amplitude and loading - for example, you might want a directional coupler to evaluate antenna/load match by looking at forward and reflected power individually.

Keep in mind that the pi's output is almost certainly not spectrally clean enough for on-air use, especially at 150 mW power levels.