Lentigo maligna is a tricky disease to diagnose in most cases, because the lesions are usually in a field of somewhat similar looking lentigines and SKs. How to differentiate them from their benign brethren? One is similarity. Does the patient have a lot of lesions that are similar in character to the one you are examining? It is very, highly, extremely, wildly unlikely that the patient will have numerous MISs, so if there are lots of other spots that look like this, then give 'em a pass.
Second is what I call the first glance phenomenon. We often look at dark lesions peripherally before we put the full fovea centralis on the spot. That "rod only" look often is a better discriminator for what is truly dark and what is less so. In other words, if it catches your eye, you should REALLY have to talk yourself out of biopsying it, or go ahead and put a hole in it. When in doubt...
Finally, get up close and personal with it. Scrape your fingernail across it. What's the texture? Smooth like a snake or crusty like a tortoise?
As an aside, I studied for a few months at the Armed Forces Institute of Pathology, one of the preeminent derm path labs in the world, and the older guys there used to call LM/MIS "precancerous melanosis" because they didn't believe those lesions were truly cancer. I'm not completely of one mind on the subject, either.