A 45 or 135 degree infill pattern on the first layer will usually reveal backlash in the XY mechanism. In a machine without backlash, the lines will be evenly spaced and just touching each other. If there's backlash, the lines will be in pairs, with gaps between every other line.
Here's a rather extreme example: https://vimeo.com/173202575
Notice that the nozzle lays down a new line as it moves away from the camera, but on the return path the line is almost on top of the previous line, leaving gaps between the lines.
