The easiest solution is to look at the L:Vars "L:var_radio1Mode", and "L:var_radio2Mode"; however the meaning of those variables can change across my aircraft. I would recommend using the variables for each specific GPS (as I've been doing more myself, as my aircraft become more complicated). Those are...
WT GNS 530:
L:XMLVAR_GPS_DISABLED_AS530_1
L:XMLVAR_GPS_DISABLED_AS530_2
WT GNS 430:
L:XMLVAR_GPS_DISABLED_AS430_1
L:XMLVAR_GPS_DISABLED_AS430_2 (does not exist in any of my aircraft yet)
TDS GTNxi 750:
TDSGTNXI_Inactive750U1
TDSGTNXI_Inactive750U2 (does not exist in any of my aircraft yet)
TDS GTNxi 650:
TDSGTNXI_Inactive650U1
TDSGTNXI_Inactive650U2 (does not exist in any of my aircraft yet)
PMS50 GTN 750:
L:XMLVAR_GPS_DISABLED_GTN750_1
L:XMLVAR_GPS_DISABLED_GTN750_2 (does not exist in any of my aircraft yet)
PMS50 GTN 650:
L:XMLVAR_GPS_DISABLED_GTN650_1
L:XMLVAR_GPS_DISABLED_GTN650_2 (does not exist in any of my aircraft yet)
KLN90B:
L:KLN90B_Disabled