Most toddlers will eat when they're hungry and stop when they're not.   Since they don't "have a lot of time" (have too short an attention span and lots of energy) to sit in one place for too long,  one good way to make sure they eat enough is to serve foods that can be eaten quickly and easily.

Parents of toddlers need to keep a few things in mind even before determining whether a child is eating enough:

1.  Toddlers don't require nearly as much food as many adults believe they do.  They need a well balanced diet, of course, and foods/drinks high in nutrition can help assure they get that without a lot effort. 

2.  Some foods are more appealing to toddlers than others.  Since they like doing things themselves, toddlers enjoy foods they can easily pick up with their fingers and eat.  They also enjoy the "challenge" of having to pick up each one of those (for example) little peas and eating each one.  Besides serving foods that happen to already be in small pieces, cutting other types of food (like bread, cheese, even pasta) into small pieces can make it more appealing to a toddler.

3.  Drinking lots of milk/formula will, on the one hand, offers much needed nutrition to a toddler; but it will also fill him up to the point where he has little interest in eating.  Eliminating a bottle/cup or two, or at least timing particular filling liquids (like milk) so they don't interfere with a child's dinner-time appetite helps.   Serving a small amount of water, milk, or juice with dinner (a very small amount) can help.  Depending on what they're eating, toddlers are often content to eat first and have their drink after.

4.  In spite of enjoying eating small, finger-sized, pieces of food; toddlers are often willing to have a few spoonfuls of foods they like "scooped in" by a parent.   Toddlers aren't toddlers for very long, and it isn't necessarily the time to worry about whether they "sit up and eat their whole dinner".   Although it takes a little more thought and effort, parents may find that "catching" their on-the-go toddler, and offering him a quick spoonful of food that doesn't take too much of his "free time", can be a good way to get a little more dinner into their child.  He can always learn to "sit up and eat" when he's a little more developmentally ready.

5.  Offering foods a toddler particular likes is, of course, more likely to result in his eating well than offering foods with less "toddler-friendly" taste.  Toddlers usually prefer peas and lima beans over green beans (which can be a little bitter).  They often like sweet potatos, carrots, or beets because they're less bitter than some vegetables.  Milder tasting cheese is something they may like more than stronger cheeses.  Fruits, of course, are sweet and usually enjoyed by toddlers.  Sliced bananas can be appealing to toddlers.  So, usually, are fruits in the form of applesauce or commercial "junior" fruits (requiring a spoon and usually a little assistance from a parent).

6.  Since "store bought" "junior foods" are designed to appeal to toddlers, sometimes a child will prefer them more than he prefers some "regular" foods. 

7.  Many toddlers prefer softer vegetables to crisper ones, so boiling fresh vegetables, or else buying canned (with no salt added, if possible) can encourage some toddlers to eat a little more.

8.  Even with the suggestion that parents allow for some "quick spoon-feeding" while a toddler is on the go,  it's worth mentioning that toddlers are sometimes too easily distracted to be interested in their lunch or dinner when something "more interesting" is going on.  The toddler who has small pieces of cheese and a handful of peas on his highchair tray may not be interested in them if he spots a parent or older sibling with a piece of strawberry shortcake, of if the dog is entertaining him at the bottom of the highchair and eating his dropped food.  Eliminate particularly "interesting" distractions when it's time for a toddler to eat.

9. Don't get into struggles over food.  Toddlers are particularly sensitive to someone else attempting to have too much control over things like food.   Toddlers can get their nutrition when parents only serve small-but-nutritious snacks throughout the course of the day, rather than expecting them to sit in one place (x number of times per day) and eat what parents believe are a "whole meal".   Again, toddlers can polish up their eating habits/manners when they're just a little older.  The point, when they're this young, is that the foods they get are nutritious.  In other words, don't make a big deal about food.  It will give a toddler "fuel" for any number of less-than-ideal behaviors associated with meal-time.

10.  Keep in mind that a few peas and carrots are higher quality nutrition than a "giant load" of white rice.  If a toddler seems to prefer his peas and carrots surrounded by some white rice he'll obviously be a little more filled up with the rice.  At the same time, parents shouldn't confuse "just volume" with "adequate nutrition".   Many toddlers may prefer the blander taste of things like white bread or white rice over whole grain, but a small amount of whole grain rice around those peas and carrots is healthier (and lots of a toddler's favorite vegetables mixed in with that small amount whole grain rice will often make it more appealing to a toddler).