I COULD tell you a couple of ways to discern what CD-Rs are what, just by looking at the serial numbers written on their hubs, but that would still require you to purchase the media before you could check it. but since you ask...I'll give you a few pointers. Please bare in mind that I only have my own experiences to go by on this, and I can't guarentee that ALL CD-Rs will follow these rules, but so far they seem pretty accurate.
Prodisc: has 2 sets of serial numbers, one is black, on the plastic hub of the disc. I havn't discerned any regular patterns for this one yet, but there may be. The second number is in the holographic circle between the main disc area and the plastic hub. This number is always 3 sets of 4 digits, and the first set of digits is usually R020. The best way to read this set of digits is to turn the disc upside down (looking at the burnable surface), especially since some brands block off the holographic hub with their labels.
Ritek: Ritek is nice to the consumer because their Serial numbers are VERY easy to recognize! They have 2 sets just like Prodisc, the inner hub one is basically useless (that I can tell), but the second one (holographic part again) is VERY useful! It will usually be something like this: RBD80M-xxxxx 80 which basically means Ritek Black Disc 80min followed by 5 numbers. On regular (non black discs) it's usually RAD80M or RAD90M (for 90min CD-Rs), and RAD74M for 74min...you get the idea.
Taiyo Yuden: These guys are EASY! Taiyo Yuden is the only CD-R that has a "crystalized" hub. So if the hub is completely transparent, it's not Taiyo Yuden.
Mitsui: Generally Mitsui CD-Rs say Mitsui in the middle of the hub. I'm still working on a regular pattern for their serial numbers. Mitsui's also ALWAYS have a protective coating on the top of the CD-R, it's one of their trademarks.
Princo: I'll get back to you on this one, I don't have Princo's Specs. Memorized yet
CMC: These guys are really easy too! again, 2 sets of Serial numbers, the first one is like Prodisc and Ritek: in the plastic hub, only the digits are written slightly larger then any other CD-R I've seen so far (though this depends, sometimes it isn't true on branded media). The Second number again is in the holographic ring. CMC is very nice to us because they always start their second serial number with the digits CD-R or CD-RW followed by the capacity rated in Minutes, and then 8 and 4 digits (ie. CD-R 80 xxxxxxxx xxxx) However, their CD-RWs are a little bit different (Different amount of serial numbers, placement of the capacity rating).
That's about everything I have so far, I hope it helps. All in all though, if you already have the CD-Rs, just run them through CD-R Identifier or cd-r media identifier (http://www.ln.com.ua/~maverick/f_cdrinfo.htm) and it'll usually tell you everything you need to know (without the eye strain)