a fine specimen of R. taurus.

You need to look at the end of the abdomen in order to sex assassins (and most heteropterans). In most assassins, the males have a bulbous capsule and you can sometimes see the parameres on each side of the last few segments. Also in some genera the rim of the last segment may be modified with a spinous process. Females lack the bulbous capsule and the final few segments look more like the other ones. In assassins the females are generally larger but this isn't usually helpful unless you have a couple individuals side by side. I'm leaning towards this being a female but I'm not totally sure since we can't see the terminalia.