I saw a checklist idea that is easy to put in place at the User Conference in Fitchburg, MA on Saturday. It also uses the mathematical emphases like Wayne suggests. Kim Schroeter from Scarborough, Maine showed us this form (one per student and per investigation): List the mathematical emphases for the investigation (listed in the book) down the left side of the form. On the right side of the form, next to each emphasis, put a 1 to 5 scale, where 1 means never and 5 means always. Leave room for comments at the bottom of the page. Throughout the investigation, based on your observations or the student's written work, circle the number that shows the degree to which the student shows understanding of that mathematical emphasis. If at the end of the investigation you have circled mostly 5's the student gets an A, mostly 4's and s/he gets a B, etc. if you have to use letter grades.
You can staple the student's work that shows his/her understanding to this form.