For me a bad day is usually signified by flip tricks just not working for me, can still knock out the basic shuv and 180 catalogue on even my sketchiest days, but if i'm not at my best it's always the flips that suffer first...usually i'll have a bad heels day so switch to kickflips, or vice versa, or suck at regular stuff so work on fakie instead.
I'm surprised generally by my consistency now at 35 compared to when I was a teen, bit better body awareness these days I suppose. the flip side to that is i'm much more fragile, missed a heelflip catch with my back foot last week and did the mini splits, i've really done a number on my thigh/groin muscles.
So yeah, bad day is usually flip tricks just not playing, or getting pitched on my ass on basic tricks multiple times... I can also usually tell a bad flip day and that I'm too tired if I primo my flips multiple times in the same session...went through a little phase of that in the summer and messed my back and neck up quite a bit from whipping to the floor.
My outlook these days is that first try is a bonus, but third try is OK. Generally it doesn't have to be a bad day for everything so if a trick I usually have is really obviously just not working I park it and work on other stuff instead.