So, the beer in the keg will not be carbed? I'm assuming a corny keg.
Pitch the appropriate amount of yeast for fermentation.
Give it enough time in the primary to ferment out and clean up byproducts.
Get that beer in the keg.
One week before you bottle, take a sanitized sample and put that into a crushed & "deflated" but otherwise intact 20 oz soda bottle. You want the bottle half full of wort.
Add a little priming sugar to the soda bottle, cap tightly and set out at room temp.
One week later, if the soda bottle has "inflated" you know that you're good to bottle carb.
If the soda bottle never "inflated" add a half package of dry yeast to the bulk aging beer and bottle.
The soda bottle thing is a yeast viability test, if were talking just a few weeks its not necessary, but I have no idea how long you'll let that beer sit in the keg.
Better option is to carb up that beer and bottle straight from the keg, which can be done cheaply with a BMBF.