This can be done by stamping the homework or marking it off in your grade book as students come into class. In order to build in more accountability on occasion, you can focus on one particular part of the assignment or ask a specific question to gauge how students did. As you observe groups working, you will be getting insight into how well they are able to share the tasks they are assigned, and can give the group as a whole a grade on its members' ability to collaborate. You may also find it helpful to have group members grade each other periodically on participation. You might have students do some self-reflection and grade themselves as participants in their groups.
Students are typically very honest.
In fact, many are too hard on themselves, so you will want to reserve the right to raise self-assigned scores. Occasionally, you may need to assign grades to projects or investigations done by each group as a whole. The simplest approach is to assign the same grade to each group member. As an alternative, you can give a lump sum to the group and have group members decide how to allocate it. For example, suppose you want to allow each student a maximum of 10 points on a given assignment, which would be a total of 40 points for a four-person group.