A way to do it would be to backup your web/css and web/js folders, then run
php app/console assetic:dump --env=prod --no-debug
again.
This way, you will have the version with changes (in the back-up) and without in web/css and web/js.
Then via a diff
command on your server, you would be able to spot the changes you have to move manually back to the assets.
If you are on a production site and really need this operation to go quickly you can do this :
The first four commands are just to have a back up of the actual state.
Then the rest is to recreate the assets as they should be and save those in a folder to compare them later on then reinstate the back up where your manual changes are so the people browsing your site won't hopefully have the time to notice anything.
cd web
cp -R js js.bk
cp -R css css.bk
cd ..
php app/console assetic:dump --env=prod --no-debug && cd web && cp -R js js.clean && cp -R css css.clean && rm -R css && cp -R css.bk css && rm -R js && cp -R js.bk js
Then you can compare the folders .bk against le folders .clean
diff css.clean css.bk
diff js.clean js.bk
And readjust your changes on the right assets
A dummy example of a diff result :
bash$ diff a b
diff a/test.css b/test.css
1c1
< .a { color:#321; }
---
> .a { color:#123; }