Just thinking that your partner is 'The One' can be really simple. But figuring out whether they really are is another different thing.
If you can't answer 'Yes' to any of these questions then your partner might not be 'The One';
1. Do they love you unconditionally?
You make mistakes, you go wrong, you do or say things that are not right. But does your partner understand that you are human and you make mistakes? Do they love you through hard and good times? If your partner's love for you comes with conditions then it's no doubt they aren't right for you.
The key to finding real happiness is finding someone who stays by you and sticks strictly to you. Are they there for you any time you need them? Do they invest their time in motivating and supporting you through the hard pages of life? Strong relationships need dedication and mutual support. Things get tough sometimes to a point where all you need is a shoulder to lean on.
3. Has your life gotten better with them?
Are you happier since they came into your life? Do you find your problems more manageable since they knocked on your door? If you find life way much better with them then they are indeed the one for you. However, if your life has grown harder then be warned.