Any way to identify "directionality" of electret microphones?
In terms of actual directionality, you'll probably just have to test them by measuring the response to a source at different angles.  Normally this would be done in an anechoic chamber, but you might be able to accomplish something by supporting the microphone on a post well off the ground in the center of a large carpeted room and walking around it with the source, or outside on a calm day.  You could tie a string to the post to measure a consistent radial test distance.

Directionality will probably be somewhat different at extremes of frequency.

Since your source probably wouldn't be omni-directional, try to be consistent for example always aiming it at the microphone.