First look at the flow chart for the project. Expand all of the elements until you find your effect.
You can double click on the layer in question to open that composition and select the layer. You can then press the U key twice to reveal all changed properties for that layer. Your "drop shadow" could just be a shape layer or a masked solid with a blur below the video layer. There are so many ways to do this that I couldn't guess what's going on without seeing either a flow chart or a timeline.
Thanks! After doing a bit of digging it looks like the comp is a previous AE file saved as a .mov so isn't editable. I've managed to replicate the effect by fiddling with the drop shadow settings.