From what I have learned, there are no definite ways to tell if a Sim is actually cheating with another. Sims cheat for various reasons depending on what they want. Some Sims cheat as a way of getting away from their partners. While other Sims cheat as a way to (through jealousy) bring their partners closer and prevent them from cheating. Still, there are certain tell-tale signs to look out for. If you are a player who watches their Sims closely the signs will be easier to detect.
Here's a few thing I have noticed:
1- Sims who are partnered in a Boyfriend/Girlfriend, boyfriend/boyfriend or, girlfriend/girlfriend will behave differently than those who are married.
2- Some Sims who are married do not wish to divorce their spouse. It's not a lack of love for their partner which makes them cheat but rather there is something lacking in their relationship which is important to them. Whatever that 'lacking thing is' they see (and want) it in another Sim. This is where it helps to know your Sim.
3- Most cheating is rarely about woo-hoo (that's a fun little bonus). Although, for those whose entire cheat is woo-hoo related, then it's all about woo-hoo. Which (in my opinion) means both Sims are seeking something more meaningful.
If you suspect your Sim of cheating or wanting to cheat, here's a few signs to look out for. These must all be done autonomously and unguided by the Simmer.
1- Changes in behaviors at home. Is your Sim pulling closer to or further away from their partner?
2- Without going into too much detail and despite common belief, icons are not randomly generated by the Sims. Pay attention to what is said and repeated between Sims. @Deshong04 has posted a perfect example of this, notice how the icons in the conversation between Fatima and Yoshi are the exact same ones in the conversation between Yoshi and Kanyin. These conversations were had at two different times. Only with Kanyin, Yoshi is denying he would (or did) 'arcade machine' with his mom. Also notice what Yoshi said to Fatima's husband when he walked through the door before he greeting him. Then notice Kibwe's reaction and Yoshi's thought.
3- Pay attention to the 'Spend Time With' wishes your Sim sends up. And, watch who they autonomously call. Eavesdrop on conversations when they are on the phone; what are they talking about? Do they mention their significant other in the call?
4- Public behavior. Normally, when your Sim is out in public, the game will 'call' those they know (or they feel they ought to know) to come to that lot. Does the 'cheat partner' show up? If they do, do they stay? Pay attention to which interactions they perform and the response. Here, body language, eye contact and facial expressions count big time.
5- Unmistakable glances and looks are also a sign of Sim desire. A cheating Sim will use their eyes to communicate with the other Sim especially when they are not seeking to lose or destroy their current relationship. Their conversation with their cheat partner will always be 'light' and non-romantic. (Flirty type traits will blow this mostly every time)
Another way Sims use their eyes are in very strong in passion relationships where the act of actually cheating would destroy the relation of all Sims concerned in an irreparable way.
