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Well, if you're physically looking at them, the red TESLA lettering on the traditional style charging posts should be illuminated when the stalls have power. Though this doesn't work on the urban style superchargers as they lack any illuminated markings, on these you just have to plug in and check. But if you are asking about how to tell if a supercharger has power when you aren't on location, your options are less reliable. Tesla's in-car navigation gets updated to show stations that are out of service or without power but it isn't a real time process and isn't foolproof. There's often a significant lag between when a station initially goes offline and when Tesla updates the location as not available. Basically, there's no guaranteed way to tell. Other avenues one can investigate would be to check for recent comments about a given location on Plugshare or in the appropriate TeslaMotorsClub (TMC) thread that is dedicated to following progress on that specific supercharger to see if anyone has been by and checked in. If you're wanting to check multiple locations, probably the easiest way to find the TMC threads for all of them is to go to the map on Supercharge.info, click on the appropriate supercharger dot and then click on the "discuss" link in the flag that pops up. Or you can use TMC's search function and restrict to threads with the supercharger location name in the title.