Csom or rest to verify user

I am uploading a spreadsheet to a task list using visual studio - the sheet will usually have users that are misspelled or incomplete and SharePoint throws a not a user error. I have to go back fix the name and then restart the 1000+ row import.

I see a lot of code to find out who the current user is - what i need is a simple routine to test if a name is a valid user in SharePoint. Preference is CSOM- but REST would be ok...

Goal is to pop up a dialog to correct the name and let the import continue...