Good way to check on the status of an FIOSystem::LoadData request?

I recently set up an asynchronous file loading system for use in my VR comic book reader, and I’m using FIOSystem::LoadData - but I can’t seem to see a good clean way to check if the read has finished (I looked through the source several times).

Right now I am writing two special bytes to the end of the buffer and checking their value in a timer, but that’s extremely hacky. Is there a better way?