I am taking over a wordpress site for a company. They don't know who is currently hosting their wordpress site, all they have is the domain name hoster and the wordpress site admin login info.
You need to check the DNS the domain name is pointed to. You may check the nameservers (=DNS) in public Whois database: https://www.whois.net/
Also, this resourse should help: http://www.whoishostingthis.com/