Don’t. Not unless you are prepared to make a proper offering. Better to let them to pass by incognito, lest they take notice of your taking notice.
A 'proper offer' does not mean money, unless it’s lots of money, and untraceable. If you don’t know what would be suitable payment, you're not ready to spot a powerful mage.
But if you do have a fortune to spare, or a preternatural object, properly cleaned, contained, and with the proper certificates of authentication, then maybe you are ready.
They are not doing magic; those are grifters and stage magicians. Powerful mages don't do magic that you can see, and don’t do it in public. They are not wearing robes. Powerful mages wear comfortable clothing that does not stand out, and robes meet only one of those two essential criteria.
Magic sets mages misaligned with our world; you will not find them by looking at it directly. They can only be seen if you aren’t distracted by the mundane. It is best to find them when you are alone; you cannot find them by attending, but only by having nothing else to attend to.
They will have a second face; one is not enough. If you are not open to this, you will not see them as they are, and may as well not see them at all.
It’s better to empty your mind of any expectation of human form at all; it will only distract.
Don’t think of the mage as a 'he' or a 'she' or a 'them'; try ‘it’.
Don’t have anything rotten on your person, although something dead may be appreciated. YMMV.
Have a totem. Have good posture. Have clean teeth. Very clean. Have an escape plan.
If you meet these criteria, precisely, turn around, it’s behind you now.