Nope. You need a proper reason to look up customer data, and “I wanted to see if they were a customer” is not one. This is even more important under GDPR, under the part where customers have a right to know what their data will be used for, and again “we will look ay your data on a whim” is not a reason even given that I have seen. Customers want to know you’re only looking at their details when there’s a genuine business reason for doing so, and many of them (in my experience) are taking advantage of GDPR by increasingly refusing to accept marketing emails.