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This is a difficult question. Depending on how well the lightning or the severity of the power surge was, will determine if the damage is visible or not. A general failure can also look the same depending on how the device failed.
You really have to take more care to evaluate of the customer is being truthful or not. If you look at the weather reports for the day, and you have a client come in with something that is not working, it is very easy to correlate the events. As for a power serge or brownout, maybe the electrical company may have logs to this effect. But, then again, under many circumstances they may have not known themselves if there was a rapid serge.
I had a modem and TV that was damaged by lightning. There were no visible means to determine this. The TV, I was able to service, but the modem had to be scrapped.
Today, we are supposed to be experiencing a geomagnetic storm from the sun. We are in Canada, and are told that in our geographic location we may be most susceptible from this. There are many companies that let their employees go home early, and they did a complete shutdown in order to not take any chances. I am still working on, because we cannot afford to close for the day. We have to give our contracts full service at any time.