You can download offlinelist from here:
http://offlinelist.free.fr/index.php?page=download.html
You can get the NDS .dat from here:
http://www.advanscene.com/
1.) Start up OfflineList
2.) Click "DAT" at the bottom of the window
3.) Click "Import"
4.) Find the DS dat
5.) Click Tools -> Options -> Renamer -> Use renamer
6.) Set the folders then go to compression and set it to 0 since you aren't archiving them then click ok
7.) Click ROMS -> add roms
All the verified roms will be moved to your verified roms folder and all the other one will be moved to unknown.
Keep in mind that this only works for commercial roms and not homebrew. OfflineList isn't perfect though, you could have
an actual rom and have it flagged as unknown because it has a different CRC than the one in the dat (This shouldn't be too much
of a problem though).
OR
You could flash your DS and run whatever you want. If it gets bricked just use the failsafe to reflash it.
(Just keep in mind if you have a DS lite if you want to unflash it, you will have to use phat ds firmware and that may cause "complications" such as
losing the ability to change screen brightness. Search around for more info on this and how to unflash.)
OR
You could just check the roms and see if it has an md5 sum of either:
8e7a3728759df265ca3a78553cf27bb8
or
a959cfa514f4c7162a81421ee99d3356
(This will just tell you if the file is taihen.nds or r0mloader.nds)
OR
Just do as seja_8 said and only get them from trusted sources. (Although trusted sources can't always be trusted. *cough*DarkFader*cough*)
