How to determine which RER B trains from CDG Airport stop at Gentilly station?

At CDG Airport train stations, both inside the train station waiting area and on the train platforms, there will be display screens overhead showing the RER B train's final destination station (usually Saint Remy les Chevreuse or Robinson or Massy Palaiseau).  Each of these names represents another RER B train that will be leaving for that destination, further and further into the future.
Underneath the next 2 trains to depart will be a scrolling list of other stations to be served (scrolling vertically up).  This is the part you want to pay attention to, looking for the station name "Gentilly".  One of the next two trains (or perhaps both) will list "Gentilly" among the two dozen or so other station names that will be served by that train.  This is how you can tell for sure that the train will stop at Gentilly station.
In the example photo below, you see "Massy Palaiseau" at the top.  This is the final destination station of the RER B train departing at 11:51 on platform 12.  The final destination name Massy Palaiseau will stay on the screen & doesn't scroll/move.  Just below will be a line of station names that does scroll up, listing all the stations that will be served by this Massy Palaiseau RER B train.  You can see the names "Luxembourg", "Port Royal" just below Massy Palaiseau.  These were just the two station names visible underneath Massy Palaiseau when I took the photo.  There were a dozen stations names that scrolled up & out of view before I took the photo and there will be another dozen station names scrolling by after I took that photo.  Gentilly will be one of the station names scrolling by if Gentilly station will be served/stopped at by that particular RER B train. 
For the RER train departing after, you can see the final destination: Saint Remy les Chevreuse (name will stay visible) and a scrolling list of stations served below.  The stations "Drancy", "Le Bourget" happened to be on screen when I took the photo.

I hope that helps.  Please post another question if that's unclear or confusing or if you have other questions.