Yes, since Landsat 7 SLC-off data contain pixels with missing data that have digital number (DN) values of 0, you can differentiate between valid and missing data pixels using a number of different remote sensing software packages. A gap mask can then be created to show where valid and missing data pixels are located.
Below is the process to to create a gap mask, using ENVI 5.2. (Please note that this does not signify an endorsement by USGS EROS, as we cannot endorse any specific remote sensing software. Other remote sensing software packages may also contain similar functionality.)