Usually smartctl -a works out of the box. But sometimes it doesn't. In that case, try using the option -d sat, e.g. smartctl -d sat -a /dev/sdb. That may solve if a modern external USB drive reports that SMART support is "unavailable" and that the "device lacks SMART capability". Because that a drive actually lacks the capability is extremely unlikely these days. If that doesn't work, you can try -d auto or consult an extensive list of options of different devices.