Electrical signals are recorded from insect taste receptors using an extracellular electrode filled with a solution containing a stimulus and a small amount of ions to conduce currents. There is no way to know which neuron is firing unless the spikes recorded are different in size or shape, or if their firing statistics is consistent with a set of minimal hypotheses. We developed a method to estimate the number of spikes fired by two neurons from the number of doublet spikes, i.e. the number of spikes distant from less than 5 ms.
See how we proceed to do tip-recording from the proboscis here (pictures taken by Marie-Jeanne Sellier, 2009).