(this page needs to be rewritten, but I go play Wolf now..)

How many servers are well populated during the peak of the day?
When it's best time to play is explained on page 2014 of this website. And that is once (or twice) a day.
Harder to describe is wtf is well populated server.. Instead of looots of text I just tell you that in this case I make the standards here B) and you should just read on. I'll explain. Ofcourse to have a good game you need a minimum of players. But imho also very important is whether you like the type of gameplay and whole atmosphere that makes or breaks it on the server. Also.. A DM game can easy be awesome with just a handfull spread over both Axis and Allied. While in OBJ servers often another 1 handfull per team would be nice. To many, a big deal too seems the location of the servers that have good games because of a fast connection (low ping), but for me that is not so important as I'm used to ping 40-150 and I see it as part of the gameplay I choose for.
So, the list below is fully from Spawnpoint of View, but accompanied with nice argumentation. Btw..
On v1.0 most servers have EU-ping but should really not be a problem in US. In recent years about 5 totally different servers (gameplay) had once a day 2-5 servers at a time. And v1.4 most servers have US-ping but neither should this be a problem in EU. In recent years about 3 totally different servers (gameplay) had once a day 1-3 servers at a time. For ECG and probably the s4ndmod.com servers you need a clean CD-key and PunkBusterID. For the others a socalled ETkey works. I put some numbers on this to give you an extra impression about activity.
In both versions there are more bot - than well playable non bot servers. In general I'm not interested in bot servers where too few real players are present on a regular bases. I always considered it a flaw that the serverlist of RtCW doesn't show what listed players in fact are bots. Ofcourse these are not in this list.
Next servers have shown months or even years of continuety and a decent admin/anticheat system:

v1.0
FUN* ([AoD] DarkMod 2.1 obj/EU-DE)
AODDM* ([AoD] DarkMod 2.1/EU-DE)
AutoMG42 (mod)
MOBdm (US?)
f* ([FF] Shotgun Instagib 1 shot 1 kill instagib by ffclan, 1.3?)
EU AODTJ* - [AoD] Trickjumping.AoDtrickjump, 1.3?)

v1.4
ECG 24/7 Beach (stats, PBobj, East Coast GamingECG US/CAL)
24/7*XXL* (obj, =TK= US)
MadOwnage (US?)
[ToaK] Greenhorn (obj+dm, about 8 different bot-servers. Download the maps before entering the servers, EU/NL)
WildWest Hang'em'High (US + sunday evenings in EU)
beach (obj, for some time a replacement for the vanished 1.0 beachserver, shrub, EU/NL)

* means 'tweaked play'.

Timezone Xploring
So u a phreak uh.. Every afternoon/evening/night first 1.0 gets active during 6 hrs and this activity shifts towards 1.4.
Especially nowadays it could be interesting to see if another version of RtCW could expand your hobby and at what times of the day. Just thinking about these timezones..
The evening of USA-players starts 5-9 hours after mine, about when I played whole evening. They themselves could find more EU players from noon till supper. And in China and Australia people live about 8 hrs earlier than in the EU. So their evening begins when I wake up to go to work. Or sleep too long because I have a day off. And when EU starts to play it's around noon there..
So maybe EU players really should try version 1.0 next to their 1.4. And wake up early with a day off to meet Australian or Eastern Russia players. Just some thoughts.

Other ways to find activity
Installing both versions of RtCW MP including the Master Patch to get the serverlist up and running, and checking at different times, would be best way to find out. (As it is important a server owner at least informs the masterlist owner :) In addition you can check out the activity of The WildWest mod with a little tool called HeH-scanner.
The most easy way would be visiting the www lists on your mobile phone, but these lists show about 90% of the servers.
And ofcourse more options exist like searching Facebook groups and active RtCW boards on the web..
Game browsers exist Xfire or Steam that can show you current serverlist of RtCW (only v1.4 afaik) but these often collect stats and most valuable seem to be the users of these tools (that are part of the RtCW community). ^ also tested QTracker which looks nice but unfortunately I found later it was pretty unreliable; in the end more than 80% of the servers didnot show up in the list.
(For serveradmins it may be handy to, next to informing the RtCW Masterservers also inform all common gamebrowsers about their server IP. (incl. QT :) that currently supports your RtCW version. In the end all benefit.)
Pathfinder is a free 1.0+1.4 server browser which is not needed because we own this übergame. But..
It's unique because it can find a playername in a server, next to the 1.0/1.4 serverlist. Nice to have next to your RtCW's. Use wolfmaster.s4ndmod.com as masterserver instead of the filled in IP. ERROR ATM?
Also, you can use it as game starter to join a server, but Pathfinder works with just one version of RtCW.