Joining the Marines, but have a dilemma? Please answer?

Long Story short, I was admitted voluntarily into a childrens mental hospital for anxiety 6 1/2 years ago. I am going to MEPS in 1 week. My recruiter knows this. He told me to just check "No". I am fully aware of the consequences of lying. My recruiter also told me my chances of getting a waiver were very slim for the mental hospital visit. I feel he is just to lazy to do all the paper work. How easy is it for them to find out? Will it come in through my background check even though it was voluntarily done with no law involved? If i stay low key, If that's even possible, As in get a low security clearance job etc..If i go through my career with no mental problems? People have said they will find out in receiving week? Don't they use the same data base for information as MEPS? I do not care to hear that the military isn't the right choice for me, Or the fact that I would be a dirt bag for lying to get in. So REALISTICALLY, what are the chances of them actually finding out?