Hi,
The Satlook Digital NIT has indeed LNB power constantly on. If you attach a regular antenna it will display an error message, complaining about a short. Not the best device for this purpose, but you will see the transponders.
In fact you will see ANY transponder operating in the supported frequency range, but bear in mind that the mentioned field meter have a rather simplified spectrum analyser capability with a large RBW (resolution bandwidth). I would guess that the Satlook Digital NIT has about the equivalent of 1MHz RBW.
This means that you can "see" a GSM, CDMA, UMTS or LTE transponder, but at a rather blurred resolution.
Also, don't forget that mobile communication is not happening exlusively above 900MHz:
You have these designed frequency ranges:
List of Mobile Frequencies by Country (GSM, CDMA, UMTS, LTE)
Common bands are around 800, 900, 1800, 2100, 2600 MHz, so regular satellite field meter don't cover them completely!
You could use a cheap SDR based radio (google RTL2832U) or the SMA/NWT range of "Simple Spectrum Analyzer" (search eBay and visit my blog:
VMA's Satellite Blog).
I think the best and cheapest option, though, is to just use your phone!
Example for Android:
https://play.google.com/store/apps/details?id=com.cls.networkwidget&hl=en_US
Regards,
Vitor