I have read that sometimes we feel hungry even when our bodies have enough food. I am trying to lose weight by eating a smaller portion of food each meal. Sometimes I feel hungry but I don't know whether I am really eating too little or if my body just hasn't gotten used to a smaller amount of food.
We have evolved in such a way that our bodies are able to store extra food. This is in part the reason we can consume more than we need, and without even realizing it. The mind only realizes that enough food has been eaten 20 or 30 minutes after the fact (The Spark, Ratey). So if you are still hungry after that period of time, then you didn't eat enough.
I think it's important to point out that fasting is not is not a long-term solution for losing weight (Why we get fat, Taubes). So don't starve yourself!
I have had tremendous success utilizing this program as of late: http://www.myfitnesspal.com/
Another thing that I do not read much about, but helps me is how I view my food. Let's say I am about to eat a bagel, I look at it and tell myself, "Wow that thing is huge! I am definitely gonna feel stuffed afterwards!" I do this at every meal and have found that my portions have decreased. For me, I believe so much of diet is mental!
Regarding whether to know if you are eating too little, I can often judge this by my energy level at any given time... especially toward the end of a strenuous exercise session. If I'm entirely more lethargic than I'm used to being that means I either got too little sleep or probably didn't have enough calories available. Since sleep is rather easy to track it makes it easy to determine if this is a nutrition problem. This is terribly simplifying the task of nutrition, since you also need to consider proper balance and quality of food, but it's a simple enough metric that you can play by ear.
Now, as for knowing if you are hungry or not, often times the signal for hunger is similar to the signal we receive when we are dehydrated. What I'd suggest doing, and what I do, is to immediately drink around 16oz of water the minute I start to feel hungry and crave food. If 30 minutes later I am still hungry, then I allow myself to eat, otherwise, I chalk up the craving to dehydration.