I can't foresee a scenario where your delving into this, let alone attempting to do something about it, will end well for you or them. Just let it go. That said, how can you tell? In my experience, it's been pretty easy to tell when two co-workers are sleeping together. They always hang out, to the point where it seems like they are joined at the hip. They have private in-jokes with one another that nobody else in the office gets. They don't work in the same department, and yet, they're always finding excuses to drop by each other's offices or cubes. At social outings, they always show up or leave together, and/or they spend the whole time in each other's company. Even in groups, their banter with one another is cutesy and flirtatious. Etc. The old rule of thumb states that if two people seem like they're sleeping together, they usually are (or soon will be). People are notoriously bad at hiding their affections, especially when the relationship is new.