How to assess math skills for Algebra I

I will be homeschooling two rising 8th graders this fall who, I believe, are ready for Algebra I.  But I want to make sure that they have mastered all the prerequisite skills first.  My thought is to assign Pre-Algebra to them first to make sure they've mastered everything.  But I'm wondering how long it will take for them to prove their mastery of this subject before we go on to Algebra I.  I don't want them to spend half a year just reviewing content they already know.  Is there a quick way to assess content and skills to see if they've already mastered them?