linux use the version number to distriguish the kernel version. but if version number is the same, then u have to use other way. for example, u save a copy of old kernel symbol table. and then use the new one to diff with old one, if have problem, then u can know u are using the new one. can check the /proc/ksyms to get the symbol table.
check u network device section when configing the kernel.
if u nic works under u old kernel. then reboot with u old kernel and see which module it use.
which nic u use? i means the chip name.