Some sushi restaurants have sushi menus from which you can order single pieces. Such menus usually have an image of all different pieces, and also shows the price for each piece.
You can then select the pieces that you want and fill out a form. Each image in the menu has a number, and the form has the same numbers. The form is all in Japanese, but the left column in there form that you can see here says "wasabi ok", and the right column means "no wasabi".
The chef will add wasabi to the pieces if you enter your amounts in the left column. You normally don't get wasabi on the side.