If you obsess about that, don't. It's further evidence that you do, indeed have OCD. But no amount of evidence will let you rest in that knowledge; your OCD will always find a way to disqualify whatever evidence you come up with. The only thing you can do is to follow the advice OCD patients have been receiving from their spiritual directors literally for centuries, back to the days when monks would come with such obsessions to their superiors- stop struggling and abandon yourself to God's arms.
Doing that is called "faith." It is never easy. It is a halting, stumbling, feeble process of false starts and failures for all of us- which can never finally fail as long as we don't give up.
And the very fact that we haven't given up proves that we have faith.
It's true that telling a person with OCD not to worry is like telling a blind man not to bump into things. But a blind man can get a cane, or a seeing eye dog, or somebody to take his elbow and guide him.
The guidance in this case is the knowledge that the worst thing you can do is to validate your obsession by arguing with it. It will always have an answer; that's the nature of OCD. Since faith is trusting in God, your question is effectively answered by the very thing that you need to do to silence your OCD: stop asking the question. Or better, refuse to ask the question. Silence the question when it arises in your mind, and cast yourself on Christ.
It will keep coming back. Every time it does, simply refuse to argue with it, and cast yourself on God's mercy in Christ. In doing so, you will not only be short-circuiting the mechanism by which OCD operates, but proving your fears groundless- because doing that is exactly what faith is.
If you don't believe, no amount of worrying will enable you to so. The only thing that will is the Holy Spirit, operating through the Word. Instead of making faith a good work which you have to perform in order to earn your way into heaven, let it be what it is: precisely abandoning any effort to earn or deserve your way into heaven.
Simply cast yourself into God's arms like an infant does with its mother. Doing that is, in itself, faith. Trying to worry your way into somehow being sure of your salvation because you've earned it by believing is not going to work. And even if you could do it, it would be the precise opposite of what the Bible means by "believing."