I think that the best heuristic is to look for bottom-lining. Have you decided on what you want to convince her of before you have determined what evidence you will selectively show her to bring her to that conclusion? If so, you might be practicing dark side epistemology.

While that sounds nice in theory, it's not realistic. In all human interaction people try to present their best attributes first. This is normal and generally harmless. In fact, most people would find it quite odd if when someone introduced themselves they instantly revealed their major self-perceived flaws. If you continue to withhold important information that you know is likely to be perceived negatively by another person over a long period then you start to cross a line that most people would consider unreasonable but I think you need to offer a more restrictive definition of what is considered the 'dark side' unless you want to rule out most normal human interaction.

It seems that 'dark side' gets used in two somewhat different ways here. What Eliezer describes in Dark Side Epistemology seems a narrower definition than is sometimes employed by others. I haven't seen a clear definition of this broader meaning but it appears to include techniques that are calculated to produce a particular effect in the audience and incorporates the kinds of 'tricks' that artists use to make their works emotionally resonant and powerful.