You can sign up for the free version of TrainingPeaks and upload your rides there automatically using Garmin Connect. TrainingPeaks will email you with your new FTP when you set your record for 20 minute power. For example, I went up Sierra Road in San Jose a few months ago and afterwards TP emailed me saying my new FTP is 337, based on a 20 minute critical power of 355.

If you haven't been riding for a while, you could lower your FTP and then raise it as TP informs you, or just do the twenty minute test.