I guess display_backlight USE flag can be used:
https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/master/chromeos-base/power_manager/power_manager-9999.ebuild#94

Looking at the code, despite its name, it seems to mean the internal display is present?
If so, we can use the same flag for  issue 865853 .

I think that my reason for not wanting to use the display_backlight USE flag directly is that it may not be accurate for unibuild devices. We may use the same system image on multiple devices, some of which have internal displays and others which don't. (I don't know whether this happens yet.)

The power_manager ebuild inspects display_backlight to set powerd's external_display_only pref on-disk, but the pref can be overridden at runtime via cros-config; see https://chromium.googlesource.com/chromiumos/platform2/+/master/power_manager/common/cros_config_prefs_source.h.

The check_powerd_config tool checks powerd prefs set via cros-config, so it may be a better choice here than checking the USE flag directly. Unfortunately, this means that the test would need to check the pref itself and return immediately without reporting errors when it can't run.

Going forward, I've been thinking that I may need to make Tast's test dependency code (https://chromium.googlesource.com/chromiumos/platform/tast/+/HEAD/docs/test_dependencies.md) support checking cros-config values in addition to USE flags.

(I believe that using cros-config still fits the goal of checking software dependencies rather than runtime dependencies, since these are characteristics of the type of device where a test is being run rather than of the particular DUT where it's running.)