I'm just changing my comment into an answer.
Its going to be hard to accuse someone of being inappropriate or sexual with just a stare. Some people just have that face or default look and might be gazing off into the distance, rather than explicitly staring. Its a pretty easy accusation to deny and you set a bad precedence that just looking at your female coworkers is going to get you into trouble.
This being said, having a straight up conversation with the accused is essentially settings that precedence. You should monitor the situation first to determine if they are spending an abnormally large amount of time staring at the coworkers, before you even approach them about the topic. Even then, you shouldn't do it directly.
Some simple strategies include
Blocking their direct line of sight
Adding pot plants or moving a computer monitor to block their direct line of sight. This will help you identify if the accused is staring at your female coworkers on purpose, or if they just end up looking in that general direction by default.
Change their seat positions
You can move the accused seat position so that the female coworkers are out of view or the accused has to make a very obvious movement to see them (e.g. if the female workers sit behind the accused, they have to physically turn around). You can also make sure they are facing away from hallways or popular paths as they might be staring when your female coworkers walk by (which could by completely non sexual and just reactive behavior to movement)
If you do plan to strike up a conversation, I would generally recommend avoiding mentioning your female coworkers. The easiest way would be to strike up a conversation about concentration and taking small breaks to stretch or move around (more relevant if you sit at a table all day). You don't want to accuse them of staring, because they will likely get defensive (especially if its a generally sensitive subject). But having a brief conversation about things to do when they aren't focusing on work will show them that someone may have noticed them staring off into the distance for a bit longer than normal.
As an example (completely fictitious)
Accused: Staring off into the distance
You: Hey, can't focus?
Accused: *Panicking at being caught. Getting excuses ready
You: You should try stretching if your having issues concentrating
Accused: *Quick acceptance of your suggestion to get you off their back
You: *Walk off
Accused: *Relief from not being explicitly called out. On high alert in case you catch them again