Time the pains. At least 5 contractions, each lasting one minute, for one hour. That's how you know it's time.
Yep, this will usually ensure that you're actually in active labor. You don't want to head in before the above is satisfied, some people like to labor at home for longer than that.
If you haven't already discussed this with your care provider, do so at your next appointment if you end up not being in labor tonight. They should have already talked to you about when they would like you to call them and/or head to L&D.