In order to identify high frequency radiation sources outside the house you need to use a directional radiation meter (the meter should have a directional antenna). The trick is to hold the meter in front of you and to scan the 360 degrees around. When the meter will be directing to, or will be closer to a radiation source, the radiation levels will usually be higher than when the source is not in front of the meter or far away from it.
If you are using a 3 axis meter it is best to configure the meter to measure only in the axis which is parallel to the axis of the radiation source (for example axis Y for mobile phone masts).
Identifying low frequency radiation sources outside the house is trickier since low frequency electromagnetic radiation is less directional than the high frequency. The search for the source should be based on getting closer or going away from the radiation source while trying to look and guess where the low frequency radiation sources are located in the near vicinity.