It can't be helped, Diana F+ only came with zone focusing so it's kinda hard to see if you're in focus (unlike rangefinder camera that had patch to see if you're in focus, or TLR and SLR camera that let you to see through the lens if you're in focus.
Also, since 120 film didn't came with it's own canister like 35 or 110 film, it is recommended to not look at film advance window and reloading your 120 film under direct sunlight to prevent a light leak (red or orangish streak on your photo)