、。
With a face that sleepy, I'd think anyone could tell.
Literal
That-much sleepy-looking face doing-[-てれば], anyone [even-でも] understand [quot-と] think [but-けど].
そんだけ is the rough/casual contraction of そんなに(だけ) 'that much' + だけ — an emphatic 'that much.' 眠そうな顔 '(a) sleepy-looking face' uses the inferential ~そう on 眠い followed by 顔 as the modified noun. してれば is the casual conditional い-dropped contraction of していれば 'if (you're) doing X.' The trailing けど 'but' softens into an unexpressed apodosis.