Is is valid to call:

ALLEGRO_MONITOR_INFO info;
al_get_monitor_info(0,&info);
printf("%i x %i",info.x2-info.x1,info.y2-info.y1);

to get the desktop resolution before the call to al_create_display()?

Try it.

It seems to work (and the documentation doesn't say I have to). So I guess the answer is yes

Now try it on the rest of the platforms.

LMAO how Ed is making user cross checking allegro5 ^^