I only know of a handful of towns off the top of my head that have sirens along the I-20 corridor. The farther west you go, the lesser the chances there are of encountering sirens. I'm guessing you won't see much at all once you pass Pecos until you reach some place in Arizona, since New Mexico towns normally do not use sirens. To my knowledge, El Paso doesn't have a siren system.
Note that these are just the sirens I know exist. Many you cannot see from I-20, however, I thought I'd include the towns/sirens, just in case you felt like going for a quick hunt.
Eastland, TX: Whelens (Vortexes?)
Sweetwater, TX: Thunderbolt 1003, Sentry 20V2Ts, and ATIs.
Roscoe, TX: Model 5 and some type of Sterling
Loraine, TX: Model 5
Colorado City, TX: 2001s
Big Spring, TX: Model 5Ts, 2T22s
Stanton, TX: Model 5
Midland, TX: ATIs, Model 5
Odessa, TX: Couldn't find any sirens; however, they must have been overlooked. The city website says that there was indeed a siren system at one time.
Monahans, TX: at least one Sentry 15/20V2T; there has to be more than this...
EDIT: Found a Sentry 10V2T in Pecos, TX. There surely are more than that...but I figured I'd add it to the list:
http://www.jmarcoz.com/sirens/pecoscity_tx.jpg