One practice that Joseph Goldstein mentions in his Sattipathana talks that I find fun and skillful is to pause for a split second when you are about to do something and you can sense that "about to" moment - that's intention, or the mental formation aggregate.
I like to try and see the 5 aggregates (mental processes) at the 6 sense doors. For example, if noting "thinking, thinking" at the mind door I could contemplate the aggregates as follows:
1. Physical/form - brain.
2. Feelings - was the thought pleasant, unpleasant, or neutral.
3. Perception - recognized it as a thought about something I had experienced (family, work, etc.).
4. Mental Formation - the intention or action to make the thought arise (its very helpful to me note intention)
5. Conciousness/Citta - was aware of and observed the thought.
I have found the 5 aggregates very helpful for understanding my meditation experience. For instance, by focusing on the breath you make the conciousness process the foreground process, and the thinking process becomes a background process. When a thought arises while you are watching the breath it really does seem like a separate process generated the thought. The conciousness process is reliable; the other mental processes not so much