There is another definition of the soul mate and as you've chosen to ask your question to a witch I'm assuming you are more interested in this concept of the soul mate from a reincarnation perspective. Generally this definition means somebody who makes regular connection with your soul over multiple incarnations. This doesn't mean they are always your lover although there may be times when the link between you is romantic. They may come back as a parent, a friend, a colleague or even someone with whom an encounter is fleeting like the stranger you meet on a train. There may also be lifetimes when one soul is resting and the soul mates do not meet.
Some people believe there is a purpose to these interactions while others believe close souls are simply drawn to one another and seek each other out in each lifetime. As for how you recognise a soul mate I would suggest they are recognisable from the benefit they bring to your life. If somebody points you in the right direction, gives you advice that enables you to progress in a positive direction or unfailingly manages to turn up with an intuitive knowledge of knowing they are needed then this could be a strong indication that this person is your soul mate.
I'd also add (and this one is from personal experience) that a soul mate will not be easily deterred. Trivial arguments and disagreements will not end their association with you. You might question their purpose in your life but you will be aware of a connection that goes deeper than friendship or interests in common and often the result is an alliance with an intense longevity.
There are many lists on the internet suggesting ways to recognise a soul mate (sadly most of them are rubbish) but there are a couple of good ideas in this one if you are after specifics.