It would help if you stated which hardware, which distro you have in mind, which distro is said to be supported and how old that hardware is.
In general, hardware is said to be supported in RH, Suse and *buntu because it is tested with those distros. No serious manufacturer will state the fitness if he hasn't tested the distro he mentiones. If the hardware is older than 18 months your distro supports it as well.
Additionally you can google for it to see if other users had succes with this hardware.