Rubrics are a scoring tool for projects. When given to students prior to a project, they clearly see what it takes to get a high score. The expectations of the project are lined out usually in the form of a table. Each critera for the project (conventions, voice, content, etc.) has descriptions for all levels, from poor to outstanding, and are scored accordingly. Once your students are very familiar with infographics you might ask them to help you design the rubric. Ask them what makes a good rubric.

RubiStar from Altec
This is my favorite tool for creating rubrics because it is quick and easy!

Another method for scoring infographics might be through the use of checklists. Altec also has a tool for creating checklists. It is called PBL Checklists. Go here to start using the tool.