As skibum says, that 20 minute is a good test (w/ blowout). 95% of that 20-min average estimates FTP.
I also do a Conconi method which goes like this:
1. Spin on your trainer at an easy pace (make sure you have a good fan on you).
2. Spin for 2 minutes at current level.
3. Ask yourself, "is this pace sustainable for an hour?"
4. If answer yes, click up a gear, and go back to step 2. If answer no, then this power level is probably slightly higher than FTP.
That's another way for rough estimate.
It's probably better to do the Conconi method first (since it's easier), then use the Conconi estimate as a target for the 20 minute method. do them on different days.