It is important to be able to open a file and determine first, if it actually is a drill file, and second what information it contains and what might be missing.

M71                   M71 = millimeters
%                     % = start of data
G05                   drill mode (as opposed to route mode)
T001                  T = tool select followed by tool number
X018600Y016200        X,Y = drill coordinates
X018600Y020200
X018600Y018200
X018600Y022200
X018600Y024200
X018600Y026200
X018600Y028200
X018600Y030200
X018600Y032200
X018600Y034200
X018600Y036200
X018600Y038200
X018600Y040200
X018600Y042200
X018600Y044200
X018600Y046200
.
.
M00                    M00 = stop program (or M30 stop & rewind)

In the above file it is clear that neither leading nor trailing zeros have been suppressed. One cannot tell the exact format of the data but you can tell that the sum must = 6. so the format could be 2.4 or 3.3 or 4.2. If you have some idea of the extents of the board you can eliminate one or more of the possibilities. In this particular example the correct format is 3.3.

The tool select command, T includes a tool number but does not include the diameter defintion. If you see a "C" in the tool command, the value that follows the C is the tool diameter.

T001C0.050