You can list of all the reactors in a drawing with vlr-reactors and determine all the possible callback conditions for a specific reactor type using vlr-reaction-name.
The vlr-reactors function returns a list of reactor lists. Each reactor list begins with a symbol identifying the reactor type, followed by pointers to each reactor of that type. For example:
(vlr-reactors) ((:VLR-Object-Reactor #<VLR-Object-Reactor>) (:VLR-Editor-Reactor #<VLR-Editor-Reactor>))
In this example, vlr-reactors returned a list containing two lists, one identifying a single object reactor and one identifying a single drawing editor reactor.
The vlr-reactors function accepts an optional argument that can be used to only list reactors of a specific type. For example, the following lists all DWG reactors:
(vlr-reactors :vlr-dwg-reactor) ((:VLR-DWG-Reactor #<VLR-DWG-Reactor> #<VLR-DWG-Reactor>))
In this case, the return value is a list containing one list. The one list identifies pointers to two DWG reactors.