Channelling is the first sign of a bad puck that baristas are taught to look for. If visible channelling occurs, then some part of the puck preparation has failed. However, successfully avoiding the large, visible channels, doesn’t mean that there aren’t smaller variations in flow (‘microchannels’) present within the puck. As we explain in this post, the larger channels may not even have as much of an effect on flavour as microchannels, except by limiting overall extraction and diluting the shot.
To date, the best tool that we have is the coffee refractometer. An even extraction is a high extraction, so the best puck preparation technique is the one that gives the highest extraction, if all other factors (such as grind size and brew ratio) are kept equal.
Naked portafilters are also a useful tool, and they can help you spot problems in distribution or tamping technique (G Davies, 2015). As well as making any large channels very obvious, it’s also possible to see variations in flow across different parts of the puck, especially during the early parts of a shot. However, as flow through the puck increases, smaller variations in flow that arise during the shot can get obscured, and microchannels are not visible with this technique — so while a perfect-looking flow from a naked portafilter is a good sign, it does not guarantee a perfect puck.
Scott Rao has reported some success in using the Decent espresso machine to diagnose channels too small or too transient to be seen in conventional ways (S Rao, 2018). This method of analysis has a lot of potential and has caused Scott to modify his own techniques considerably.
All of the above methods rely on actually making the espresso, to indirectly assess how good the puck preparation was.