Just to disagree a little. The concensus is generally to burn at the rated speed of the media as the burner firmware gets optimised for this speed. That should always be your starting point. Often the media can be burned at a higher speed and sometimes it might be necessary to drop the speed below rated but not often. Burning at 2x does not guarantee a better burn & mostly will produce a worse burn. In fact some burners will not drop that far anyway.
The problem identifying media is that it's the Media ID that's important ( the dye the blank was made from) & you can't be certain that X brand will always be that ID & until you've opened the pack you don't know.
Depending were you are in the world there will be some suppliers who can generally be relied on to supply consistent media. In the UK it's probably SVP & sometimes APR Media.
If you're using +R media then if your burner supports bit setting then setting DVD to DVD-ROM will generally prove more compatible with standalone DVD players, certainly older ones.
What burner are you using & what burning program?
You can get Nero CD/DVD Speed that can identify the media id. http://www.cdspeed2000.com/