I would give the same advice as Bob: try to sing a wellknown minor or major song to the given chord.
Make a selection of only 4 songs: 2 with an up-beat of a fourth in each mode.
The song that fits will give you the answer.
You can even try to make the test with only one song:
HAPPY BIRTHDAY TO YOU or A GROOVY KIND OF LOVE, FRERE JACQUES
(if one fits it's a major triad, if not it will be certainly minor)
Now you can make the control-test by a minor song (scale or minor triad)
the Themes of alla Turca, Moldau, Lady in Black, El Condor Pasa
If you know Grieg "PEER GYNT"
make this test with
In the Hall of the Mountain King (minor)
Morning Mood (major)
Take care with the Moldau (Smetana) and In the Hall of the Mountain King (Grieg): Both tunes are modulating to major, but maybe that just this modulation to the related key will help you to hear and learn the difference for identifying the different modes.
Maybe this link can help you too: