Yes you can tell right away in the field if you've got some good stuff. The core is loaded into core boxes and then carried over to a core shack for the core loggers/geologist to work on. They run a hand held gamma ray detector over the core to check for anomalous radioactivity. The readings are in counts per second(cps) which can't be translated into grade but anything over 1000 cps is good and over 10,000 means your most likely in the % range for grade. They also run a downhole tool that measures gamma radiation although the hole condition may prevent this. A more specialized device called a spectrometer ( which can be hand held size) can separate the gamma rays into different energy levels including the uranium window but this device is not always used as there are few other possibilitites that could generate a high count rate. More detail is described in this press release from Hathor when they made their find in February 2008. http://www.hathor.ca/s/NewsReleases.asp?ReportID=342755

They released information directly from the field before any laboratory assays were done because it's very difficult to keep a find secret. I'm going to risk a little humour here but I think it clearly illustrates why you can't keep it a secret. Imagine your a driller or rig hand and you see some strange behavior by the core shack, or a courier showing up at Kodiak's offices when you also see the same strange behavior. They will know that they should call their brokers and put in a buy order when they observe this behavior that starts at about the 1 minute mark of this short clip:

http://www.youtube.com/watch?v=stHUypwr-V4