decoupling cap srf
You can use an ac coupled oscilloscope on the DC bias line to see what is there. YOu can also use an ac coupled probe and a low frequency spectrum analyzer, but you have to be really careful to not blow up the spectrum analyzer when doing it, so you one do it if you are really looking for low level ripples.
Often there are switching regulators used, and they have a big ripple component that needs to be filtered out. Sometimes you need CLC or CRC multisection filters in the bias line.
For an LNA, probably a 1000 pf in parallel with a 1 to 4.7 uF will be all you need. If it were a power amp, you would need lots more, especially if it was a difficult waveform (OFDM, QAM).