How do I figure out how much equity I have in my car?

Your equity is your car's current value minus what you still owe on it. Get the exact payoff amount from your lender (not just the balance on your statement, which may exclude daily interest), then estimate the car's value with a few trade-in and instant-offer quotes. If value is higher than payoff, you have positive equity to put toward your next car. OffLot's Trade-in and Sell my car pages are built around this exact math: estimated value minus payoff equals your trade equity.