How to Know If a Piece Will Look Good Post Tailoring

I have an hourglass shape on a petite frame (big bust small waist), which means most. Clothing items have to be tailored. I know what sort of pieces to look for in terms of neckline and length, but struggling to know while in the fitting room whether the piece is worth buying and will be perfect post tailoring.

What sort of things should I be looking for to know if the item can be tailored? What are stores that have quality fabrics that are worth tailoring?