My rhubarb pie is runny
The answer from zerobane provides some reasonable suggestions for working around a few common problems. Of course, they require you to know you're going to have problems ahead of time, which is a problem if you're using a &quot;tried and true&quot; recipe that fails infrequently... So ideally, you find a way to identify these problems before you start making your pie, and adjust the recipe as needed.
Water content
To gauge the water content, macerate your rhubarb ahead of time:

Cut the stalks into small pieces
Mix well with sugar - most pie recipes call for a rather large amount of sugar in the filling, so add it all now.
Refrigerate for several hours.

The sugar will draw out moisture from the rhubarb, allowing you to estimate how much thickener you'll need before you start cooking. Which thickener you use will depend on the style of pie you're aiming to make. You could also pour off some of the juice at this stage, but this will tend to reduce the rhubarb flavor.
Pectin content
The other variable here is pectin. A high pectin fruit (like the granny smith apples zerobane suggested) will - when mixed with sugar - happily thicken its own juices. Pectin levels in rhubarb vary, but generally it is fairly low compared to self-jelling fruits such as apples or oranges so you'll generally always add some additional thickener or (if you're making a jam) additional pectin; adding apples accomplishes the latter, while tapioca flour / clearjel / corn starch accomplish the former. Note that if you opt to add grated apples, you can press the juice out of them first (press them into a sieve or wrap in a towel and squeeze) - this'll increase their ability to thicken the rhubarb juice.
If you were hoping to depend heavily on pectin to thicken the filling, you could estimate that ahead of time too. But for a pie filling this is probably overkill; as zerobane suggests, simply pre-cooking the filling (with thickener adjusted based on the liquid released during maceration) will quickly tell you if additional assistance is needed. To add additional starch-based thickener to the hot filling, first mix it into a small amount of water (or fruit juice, or reserved rhubarb juice) and then slowly pour it into the filling while stirring; then simmer for a few minutes until the filling reaches the consistency of gravy - remove from heat, pour into a pie shell, and bake to finish.