Are you getting users logging in and potentially trying to defraud or break in ? You may wish to pre-approve registrations.
There is a setting in v3 in Customers -> Customer groups -> Edit Group -> Approve New Custoerms.

Set to yes to allow admin to approve customer registration.

Normally you can tell immediately if a users is suspicious, hwover you cna check teh IP address in google, type IP. e.g., 192.168.1.1 and if it has been reported in a fraud situation already you will likely see several posts on that IP identity.