There are various methods to check the current resolution.
compile modetest from Mesa (not sure if there's any package including it) and use it to inspect the current resolution. https://gitlab.freedesktop.org/mesa/drm/tree/master/tests/modetest
there's a similar utility (kmsprint) in the kmsxx project - https://github.com/tomba/kmsxx
Both utilities also allow you to test various resolutions to see if they're supported by your monitor/display.
SDL2 has specific support for mode change in their KMS/DRM video driver. You can see how it's done by looking up in their test folder for various testing utilities, most of them support the --fullscreen parameter (when they set a fixed resolution, usually 640x480) and --fullscreen-desktop (where they use the current 'desktop' resolution and scale the main window to that).
Note that none of this is specific to Armbian, the behavior and capabilities are dictated by Linux's DRM/KMS APIs.
xrandr works only withing Xorg, so it's normal it's not working outside of it. There is no equivalent to it when you're using a pure KMS/DRM environment, the resolution change/switch has to be done at the application level.
If you want to set a specific start-up resolution for your system, instead of Linux choosing the best resolution from the ones reported by your display, then you can use the video Linux kernel parameter to set it up - see https://wiki.archlinux.org/title/kernel_mode_setting#Forcing_modes.