If I remember correctly, the spec is that you climb to around 2,500 feet, full flaps, gear down, 100 knots, flight idle, and check the descent rate. Should be somewhere between 2000-2500 fpm. Don't be freaked out if one or both beta lights come on.
That is all from memory, and I could be completely wrong, but I've done a bunch of those flights in the past.
Personally, I prefer the flight idles to be near the bottom of the spec (i.e. close to 2500 fpm descent), but be aware that this can lead to potentially very high descent rates near the ground, and that can be very dangerous. See the accident we discussed in the General section...