I'm working on a project at the moment where it would be really useful to be able to detect when a certain topic/idea is mentioned in a body of text. For instance, if the text contained:

Maybe if you tell me a little more about who Mr Balzac is, that would help. It would also be useful if I could have a description of his appearance, or even better a photograph?

It'd be great to be able to detect that the person has asked for a photograph of Mr Balzac. I could take a really naïve approach and just look for the word "photo" or "photograph", but this would obviously be no good if they wrote something like:

Please, never send me a photo of Mr Balzac.

Does anyone know where to start with this? Is it even possible?

I've looked into things like nltk, but I've yet to find an example of someone doing something similar and am still not entirely sure what this kind of analysis is called. Any help that can get me off the ground would be great.

Thanks!