Visual. Basically, if it looks like enough water to paddle at the put-in, then it is good to go. If it looks low, it will be low. The slides are OK when the bouldery stuff at the beginning and end are no fun. In January of 2007, the water came to exactly to the bottom of the river right bridge footing at the put-in, and this was a slightly higher than minimum level.
The two rain gauges that are most directly relevant are the Grandfather Meadows
and Edgemont gauges
. I have seen it run with two inches in twelve hours, but I would consider that a minimum. At the takeout, you are looking at Lost Cove Creek, which can have a much different water level. I have seen the Gragg Prong really low when Lost Cove was high and vice versa.