Insert check mark graphic in FCP project.

Lots of way to do this, maybe one of the easier is to get a checkmark from the text tool. In the lower right corner of the viewer, click on the generator button and from the drop down menu choose Text > Text. Edit that text the the video track above the video you want the check mark to appear.

Once you have done that, double click on the text clip you have just placed in the time line and in the viewer click on the controls tab. Highlight the "Sample Text" in the text box and type Option + V to generate a check mark.

You can use the controls to make the check mark as large as you need.

One you have it sized correctly, you can you the motion tab to position it where you want.

If you want it to look like it is being written on, then keyframe the crop controls so the the check mark starts cropped completely off by shifting the right crop control over so that the checkmark disappears. Set a crop keyframe there. Then move the playhead 15 to 30 frames later and move the right crop control back to its original position so that the check mark is revealed. Put a keyframe there.
Now the checkmark should look like it is being written on.

MtD

Webdings font I think has some checkmarks. You could use an animated garbage matte to wipe it on. If there's a lot of this maybe easier in Motion.

Tom's thoughts are good. I've done that for a lot of game show pilots, I generally just hand draw one in Photoshop and bring it over - then it's easy to animate on using key frames in the motion tab OR using transitions.

But, finding a font with a checkmark is a very good and easy solution.

pJ