How can I tell if my child is too young to understand me or if he is just ignoring my directions?

Test with fun stuff. When toddlers have enough word understanding they are likely to respond in general. Try to avoid mixed messages. Parents use nurture speak "do this, ok" and parent speak "do this". If you slip up and use nurture speak you are asking a self invested todler for permission to parent.(ie is it ok with you if you pick up your toys) nurture speak is fine when asking them to try a new food.
Test with fun stuff. The easy answer is to try something that he will want to do first. For example, "get the cracker on the table" when he is hungry will help you determine if he can follow directions yet. If he understands the fun stuff but not the "un fun" stuff, he may be ignoring you.