Last post I referred to how a given motives can drive different and non-obvious actions, such as Achievement driving a salesperson to do influence-related behavior or Influence/Power driving friendly, affiliative behavior. The question then becomes: "How do I understand what really motivates someone?" Good question, especially since most people can't accurately identify their own motives! There are at least some answers below...
People are complicated, especially adults, who have had time to learn appropriate behaviors and generally have enough self-control to do things that are not driven by their emotions. Motives are relatively simple and very deep. If they weren't so powerful (emotional energy!) they'd be hard to see at all. If most people can't spot their own motives, how on Earth (or off, I'm not picky) do you spot others' motives?
1. Look for those behaviors that are closest to the motives themselves
2. Look for feelings and energy.
3. Look for how they persuade you
Looking for Behaviors
When someone does something they cannot explain, it is often motive-driven. The best test of this is when there is something else important to do. If it comes to a showdown between "important" and "fun," especially under pressure, a lot of people go to fun. This is part of the "why am I playing a video game the day before something is due?" phenomenon. Under emotional stress, people act out of their equally emotional motives--the self-control can't manage to bottle the pressure.
This is sometimes true even when not under emotional stress--if the important thing isn't close enough yet, it doesn't have enough strength to "out-vote" the motive.
Then the issue is down to "why is the person doing this?" Fun often means motive, but which one? Then you have to think hard. I referred to a manager micromanging--do they micromanage because they want to control a person, or just because they want to make sure the task is done right (right in this case meaning their way)? You have to be careful not to confuse your own reaction with their intent. If they seem blind to your feelings, that is a clue that perhaps their Power/Influence motive isn't in gear. And I mean "blind," not "deliberately ignoring." That's different. Cluelessness to impact on others is different, and often associated with people who have primarily Achievement motive, and little else.
Looking for Feelings and Energy
When do people light up? Or, alternatively, when do they get frustrated and anxious? Both relate to motives. If you are speaking to someone and they suddenly perk up--eyes brighten, they wake up, their pupils dilate, they get more energetic--that's a clue! Or when someone expresses frustration, what are they frustrated about? "I can't get this to work right!" versus "I can't get his attention on this!" versus "She turned on me!" What's the resentment about? What hurt their feelings?
Researcher Wei Chen found that asking people items about their frustrations actually worked better than items on what they liked. I can imagine several reasons for that, not least of which is that when something fits our values, we may call it satisfying. But negative emotions are often more precise.
Looking for How They Persuade You
Simple: people persuade in a way that is persuasive to themselves. That means they intuitively go for their own motives.
Achievement: "but it's the best way!" "I've worked it out--it's more efficient!" "But this saves time!"
Affiliation: "we're all counting on you!" "It'll hurt XYZ's feelings!" "But what about the team?"
Power: "Everyone will notice!" "Think of what a difference it'll make to everyone!"
There is one important exception to this: Power/Influence motivated people are often more tuned in to what will influence you, so they aim at your motive. It depends on how skilled they are at influence techniques and empathy, of course! (Maybe next time I'll talk about learning empathy...). But if someone is aiming at multiple motives, I'd guess they are power motivated, and trying anything that works. There was an excellent illustration of this in a Voyager first-season episode (#7?), oddly enough, when they were trying to persuade a Romulan to let them transport through a tiny wormhole.