How would I write an app to find a 4 leaf clover?

I'm a reasonably competent programmer in several different languages. I'm not asking about how to write an app, but I've never trained a neural net, which I assume I would have to do. Then I would have to somehow break up an image of a field of clover into discrete units and test each one to see if it matches what my net thinks is a 4 leaf clover. Where would I start? Thanks.