315Mhz?, you can use a RTL-SDR radio. they're cheap ($5 to $30 in the USA ) and they not only can tell you if a module is transmitting, but you may even be able to see what's going on between the devices.
more info available
[here] and they sell RTL-SDR radios too, some of their bundles contain antennas, cables, and other accessories. they also have links to other companies that sell radio dongles.
i don't know where you live, but 315Mhz devices are allowed in ITU region 2 (the Americas) but not in region 1 (Europe & Africa). i'm not sure about region 3 (India, China, Australia, etc...)