I start by plucking the feathers around the vent area and cleaning it really well with Chlorhexidine. Then I stick an 18 gauge needle in about 1/4", attach 35 ml syringe and draw out fluid. If the needle is placed correctly the fluids will start dripping on it's own. Picture:
Note that this...
If it feels like a water balloon, it is ascites, but hard to say what's causing it. There is no "cure", but draining her might buy her a few extra months.
-Kathy
X2. I had one that lived quite happily for 9 months after one draining, but most others weren't as lucky. Eventually they will all succumb to the disease process that's causing the fluid build-up. Please do the finger in vent exam and rule out egg binding before you do anything drastic.
-Kathy
If you are *100%* sure it's just liquid, and that there no masses, you could try draining it.
Study these pictures, then get a glove, some KY Jelly and gently check for an egg.
-Kathy