say ur barb fishing and have sticky keys(left on) set to the shift button so after a full inventory all u need to do is left click, is that allowed?

I think you should be fine if you're doing the clicking urself.

you'll be fine mate 

i hotkeyd my shift button to the spacebar button and resting my left hand on it. How can they tell between that or just leaving sticky key function permanently on while fishing?

Likely yes due to unfair advantage. But proving it a whole different story, let's be obvious though if you do it way too much then expect it.

Yeah man youll be fine ive even ran ghost mouse keys for theiving on multiple accounts I doubt you would ever get caught and I dont think its bannable cause its a windows feature