That depends on WHY you are unable to view the tooltip. I could be that another window is blocking it, or that focus has been taken by another process, or that the window with the "tool" that should have a tooltip is incorrectly placed, or that the programmers dis a poor job and misplaced the tooltip (or forgot to make one)... there are hundreds of reasons.

tooltip

you hover the mouse over the control in question.

ToolTip or ScreenTip

Within the game, you can mouse-over the item to bring up the tooltip, which will show empty gem slots near the center of the tooltip (below the stats, but above any green text). You can also bring up the tooltip if you click on a link to a weapon, if it's been linked in a chat channel.

Outside of the game, there are a number of websites that contain databases on WoW items.