If I am caught with an item that is concealed, but have not left the store, can this be considered theft

Depends,I think in some states that is considered theft or at least attempted theft.

In NY, yes. All they need is intent. Usually store security waits until you pass the registers but I've seen people stopped before that and charged.

It's difficult to explain the merchandise in your purse or coat pocket, particularly if other merchandise is in the cart.

And, yes, those charges have stuck.

It really is a sticky situation. Generally the intent is the compelling factor. Of course, you can always argue "but I was on my way to pay for it." And for a criminal conviction, even a minor one, you have to be proven guilty "beyond a reasonable doubt." So if you claim that you "were going to pay for it" I don't see how anyone can prove otherwise beyond a reasonable doubt ; one can't prove another's state of mind, try though they may. But to be on the safe side, don't do anything that even looks suspicious (like putting a piece of unpaid merchandise in your pocket or purse) and don't expect the system to go by the book in such a situation.