You can check if the costumer is a taxpayer for e-invoice or e-archive from the links below.
E-invoice inquiry;
http://www.efatura.gov.tr/efaturakayitlikullanicilar.html
E-archive inquiry;
http://www.efatura.gov.tr/earsivsorgula.html
As you already know, if a taxpayer is a taxpayer for e-archive, they are also taxpayers for e-invoice.
But e-invoice taxpayers are not taxpayers for e-archive.
On this lists, the searches are performed through business firm names. You can access customer's business firm name through crm as shown below.
You can find it from the LOGO CURRENT ACCOUNT section under account.
You can get help in every situation you are not sure of from the portal crew.