How do I perform the equivalent of the TestN operation in /free?

Here's what I'm going to try to do:

D fieldAlpha S 7A
D fieldNumeric S 7S 0

fieldNumeric = fieldAlpha;
On Error nnnn;
(Error handling goes here)

A couple questions/comments:
1) Is this the proper way to do this?
2) The only valid test would be if the GT ind were turned on. In other

words if this were non-free:
gt lt eq
C TestN myAlpha 31 32 33
Only if ind 31 were turned on would the TestN be a valid result -- no
spaces allowed.
3) So... I need to identify the error code(s) that would be returned
(i.e. nnnn in my "On Error" code above) for:
* Field is not numeric
* Field contains blanks

Thanks in advance!