This calculator works well:
http://www.mrgoodbeer.com/carb-cal.shtml
And if you're really serious you can send a sample to this lab:
https://oregonbrewlab.com/product/carbohydrates/
Ignore anybody who says calories are all that matter. Anybody that has been successful with Keto knows how it works.
I've done keto on and off for years now and I've been using AMG in a lot of my beers. I started using White Labs Ultra Ferm but I found I get the same results from using dry...
For what it's worth, that Mr Good Beer calculator is somewhat accurate. I've been doing Keto and experimenting with Brut'ing all my beers and had one tested by Oregon Brew Labs (I was in Portland on a trip) and it tested at 3g carbs exactly as the calculator predicted.