The information about the layers used for routing are available in the technology lef file. Please go through .lef file.
You may see a commented section like this in the .lef file.
# [Layers Information of 7layer]
# #layer layerNmae pitch width spacing direction
# layer7 METTOP 1.68 0.84 0.84 H
# layer6 METG1 0.84 0.42 0.42 V
# layer5 MET5 0.28 0.14 0.14 H
# layer4 MET4 0.28 0.14 0.14 V
# layer3 MET3 0.28 0.14 0.14 H
# layer2 MET2 0.28 0.14 0.14 V
# layer1 MET1 0.28 0.14 0.14 H