If your shower will not hold a steady temperature or the handle has stiffened, that is the valve behind the wall rather than anything you can see.
You may not need to lose your tile
Worth knowing before you agree to anything. A lot of valve repairs can be done through the existing trim plate, or through a small opening in the wall of the room behind — often a closet. Before accepting a quote that involves taking out tile, it is entirely reasonable to ask whether the valve can be reached from behind.
Sudden cold or hot water
If the shower runs cold when a tap is used elsewhere in the house, the valve has worn. It is more than an annoyance — it is how people get scalded, so it is worth attending to.
Water pressure on a well
If your shower pressure fades away rather than staying steady, the problem may not be the shower at all. A pressure tank that has lost its charge, or a pump struggling, shows up in the shower first because that is where you notice it.
A stain on the ceiling below
Usually the shower base, the drain connection or grout letting water past. Three different repairs, and identifying which before opening anything is the difference between a small job and losing a bathroom.