You don't. It may take quite a while before you'll know, or you may never be sure.
Don't put such an intangible burden on yourself. Every decision you make is the right decision if you take the perspective that you are living life, being human and learning along the way.
Make a choice. Be happy with it. If it goes well, great. If it goes poorly, learn from it and make your next choice.
When I reach a crossroads I listen to my heart. If my heart screams at me "What the hell are you doing?" I turn away from that path and try another. If my heart simply hums and thrums with happiness, I know I am going the right way~^^
It's always hard to know for sure if you have made the right decision...I guess it depends a lot on circumstances as well.
I usually listen to that little inner voice. Although sometimes I have ignored it, and have fallen flat on my face. Just goes to show, that your first gut instinct is probably the best way to go.
I have a Quote:
When u Face Choices,
Just Toss a Coin,
Not bcos it settles the question,
But while the coin is in the air 'You will know what your heart wants'.
When you think it is the right decision, dare to go with it. If it was the right choice, the picture becomes clearer and things fall into place. You get the feeling that this was "meant to be". And you will feel at peace. All the best!
Is it a sin? no -- go for it! yes -- don't do it.
There is no right decision, go with your gut and end up where you end up.