Here's a worksheet on reduced relative clauses for you to check your students on reduced relative clauses. In the worksheet, students first need to decide whether the relative clause can be reduced or not, then make the necessary changes in the sentence to make it shorter.
Students usually confuse the omission of relative pronouns with reduced clauses. It might be a good idea to elicit or teach the difference between them before handing out this worksheet.
You can separate the class into two and hand out A&B versions evenly. Then, ask them to swap the two worksheets in order the students to check each others.