There is no easy way to take a "risk" to find out if a person wants the same thing you want especially if you are becoming emotionally invested.
If a person is uncomfortable with bringing up "our relationship status" the best thing to do is (ask him/her if they are dating other people.)The answer to this question will let you know where you stand.
It also allows both people the opportunity to state if they want to date exclusively. You might also ask (where do you see us 5 years from now.) If there is any hesitation you'll also have your answer.
What does "the next step" mean?
A 18 year old who has been dating someone since they were 16 should not be thinking egagement/marriage is the "next step".
With regard to marriage it's not about "time" as much as "who" "where" and "Why".
The "who" (is this the person you really want to spend the rest of your life with?) Do you share the same goals?
Are they financially stable, mature, honest, trust worthy, attentive to your needs...etc
The "where" ( Are you in the right place at this point in your life to take on all of the responsibilities that come with marriage and a family?) Have you completed your education, began to build a career, paid down your debts, have enough life experience to know YOURSELF and what you truly want in a mate.
The "Why" (If things are great the way they are what do you think will improve by going to the "next step"? What will the step after that one be? You don't go to the next step simply because you want a "new title" or you've been dating the same person for a certain amount of time, all of your friends are getting married...etc
Marriage is not the be all end all.
Too many people think once they're married they can "relax".
They stop doing most of the things that caused their mate to fall in love with them. Marriage requires the same effort as dating.
Commitment does not mean it's ok to take someone for granted. In any relationship we are either neglecting or nurturing. It's easier to maintain a fire than it is to reignite a spark.
Don't think that by going to the "next step" life gets easier. A happy marriage is a real "labor of love."