i just want to know , ob westhost a reseller is. they wrote me that they have their own servers. but the dns query looks like :

DNS Servers:
westhost.net name server ns.host4u.net
westhost.net name server ns2.host4u.net

how can I really deduce if someone is just reselling services?

WestHost is not a reseller, they lease/co-locate their servers in a NOC owned by another company, most consumer web hosting companies do this, otherwise they would have to pay excessive amount for their own connectivity.

I believe Host4u.Net is owned by OLM. Host4u.net is the DNS server for many, (possibly hundreds) of web hosting companies. I believe it is a shared resource between all hosting companies that share the NOC. This should make sense as one DNS server can handle hundreds of thousands of entries.