A styrofoam cup 8-10 oz works okay as a hack method. Shoved into the grouphead of a La Marzocco (portafilter removed) and then stick your milk thermometer (or a glass one if you have it) into the side of the cup. Run to get shot volume 1.5-2 oz or so and then you watch the thermometer for its spike.
This is a hack job for sure, but, get's a decent reading on a LM style double boiler machine for immediate diagnosis of a severe problem. I have a photo (but's on my phone and I need to bluetooth it in). There are much more specific resources out there for this method. See home-barista.com or this thread in Coffee Geek. Also, Espresso! My Espresso has a link in the right hand column.
use the fastest updating thermometer you can find or already have. the fluke is the most common and links up with scace, modified portafilters, etc.
to check without a portafilter mounted thermocouple the easiest way is to remove the screen and screw and run the group. you'll see the water pours out in one single stream. if your thermometer is a digital handheld it should read fast enough before the temp starts quickly dropping due to the zero resistance factor. you'll have to hold the tip of the probe in the stream of water as close to the screw hole as possible to get any sort of decent result.
i've done it this way in a pinch. not entirely accurate but enough to get a good idea of the temp coming out. you'll probably get 5-7 seconds of flow before it starts dropping fast.
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