Sample Salesforce Custom Permission:

Sample Code:

Boolean accessBool = 
    FeatureManagement.checkPermission(
        'Create_Account'
    ); 
System.debug(
    'accessBool is ' + 
    accessBool
);

( Or )

Boolean accessBool;  
Set < Id > permissionSetIds = new Set < Id >();  
List < User > userList = new List < User >();  
for ( SetupEntityAccess access : [ SELECT ParentId FROM SetupEntityAccess   
                                   WHERE SetupEntityId IN ( SELECT Id  
                                                            FROM CustomPermission  
                                                            WHERE DeveloperName = 'Create_Account' ) 
] )   
    permissionSetIds.add(access.ParentId); 
     
userList = [ SELECT Username FROM User WHERE Id IN (  
        SELECT AssigneeId FROM PermissionSetAssignment 
        WHERE PermissionSetId IN :permissionSetIds AND AssigneeId = UserInfo.getUserId() ) ]; 
accessBool = userList.isEmpty() ? false : true;