Hi,
There are multiple ways. For example, the person can go to their Learn profile --> Transcript tab and there is a button to create a link to share their transcript. They would send this link to you and when you clicked it you would see the person's transcript with certification(s) right here on learn.microsoft.com so you know it is legitimate.
Another way is to post a question on the Microsoft Certification forum:
https://trainingsupport.microsoft.com/en-us/newthread?threadtype=Questions&forum=mcp
Still another way would be to look at the person's Credly profile which should have the certification and exam badge(s).
If the above was helpful please click Accept Answer.
Thanks.
-TP