Case the joint

[Dark Angel]

I know the meaning of this, but can it be also used as slang to say that one was looking around shops before deciding where to buy from?

[Quizmonster]

Not really. To case the joint/gaff/job means to study the layout of premises before robbing them. As you are planning to buy the goods rather than nick them, the phrase doesn't apply. It has been used as purely criminal slang for the last hundred years or so.
I suppose there is no reason why you shouldn't use the phrase jokingly, but I'd advise you not to do so within earshot of an officer of the law!

[jno]

sure, go ahead. Inventive twists on old phrases are what keep the language alive.

[HeratthePC]

I agree with jno - you go on ahead and case that joint!

[Quizmonster]

I totally agree...only he/she wouldn't truly be using it as 'slang' but only 'inventively' (as Jno suggested) or 'jokingly' (as I suggested).