In case of Linux with glibc, check /etc/nsswitch.conf. The line in there starting with "hosts:" has the configured order of host name resolution methods, where "files" means /etc/hosts, and "dns" means /etc/resolv.conf. Other entries, like "mdns" or "mymachines" are additional optional modules and you should consult their respective documentation, and you can always uninstall them if you don’t want them.