Lift angle is "The angle of rotation of the balance in which the roller pin is in contact with the pallet forks." (Credit: answers.com)
Along with beat rate, lift angle is the second required bit of information in analyzing the performance of a watch movement. Given those, you can calculate balance wheel amplitude, beat error and rate performance. See Brian Mumford's nice explanation here
You can generally find lift angle on the spec sheet. If not, check this google docs spreadsheet to see if you can find it there.