What we USians call a "check mark" can be found at character 252 in the
Wingdings font and at character 80 in Wingdings 2, along with various
checked "ballot boxes." In Word 2007, click on Symbol on the Insert tab and
choose More Symbols..., which opens the familiar Symbol dialog. Then choose
the Wingdings or Wingdings 2 font.
Note the panel of "Recently used symbols" at the bottom of the Symbol
dialog. When you first install Word 2007 (at least when it is not an
upgrade), this panel will be populated by what Microsoft presumably thinks
are useful symbols. As you add your own by selecting symbols in the Symbol
dialog, they will replace Microsoft's default choices. This is important
because these are also the symbols shown on the palette when you click
Symbol on the Insert tab. Once you've inserted a check (tick) mark, you'll
see that it has been added to the palette.