How do you determine if a plant is autotrophic or heterotrophic, and are there any traits in particular that can be used to classify species?
This in part depends on how you define heterotrophic. The most common definition is the inability to synthesise energy-containing molecules, most commonly by fixing carbon through photosynthesis (but dont forget chemotrophs). However, I've sometimes seen definitions that also include dependency on external sources for nutrition (e.g. nitrogen) but also to supplement carbon sources.
Using the first, main definition a lack of chlorophyll is a sufficient trait to define heterotrophic plants, which clearly places Orobanche (lacks chlorophyll) in the heterotroph category. If you use the second definition you will include more species, but it also becomes much more tricky to draw the line between autotrophs and heterotrophs. I know that Melampyrum pratense is a hemiparasite, but don't know exactly to what extent, and therefore cannot say if it is accurate to label it as heterotrophic (using the second looser definition - it has fully functional photosynthesis). Checking whether a hemiparasitic plant can establish, survive and reproduce without its host should be a useful way to establish if it is a obligate or facultative hemiparasite (I suspect that Melampyrum pratense will be found in the latter category). Myco-heterotrophy can also be obligate and facultative, and can involve transfer of both carbon sources and nutrients, see Merckx et al (2009) for a review. Insect-eating plants such as the Venus flytrap are however sometimes called partial heterotrophs, based on the fact that they get part/most of their nitrogen from insect trapping, while still obtaining their energy from photosynthesis.