First off - Welcome to Bike Forums.

Your question is a little too generic and so you suffered the humor/wrath of some of the Gent's here.

Joe Friel has written some good books and I've one on Power Meter training in front of me right now.
He offers a method by which to estimate FTP using the following factors.

Your lean weight (as in not your actual weight, but what you should weigh)
Sex
Age
Altitude
Number of days per week you train

But this is still just very rough numbers.

If you use a Heart Rate Monitor & suitable software/hardware then Strava will actually give you estimated power for you rides.
It's a cheap way to get an indication without the investment in a Power Meter.

All the best

Barry