[AccessD] Increment or replace?

Dear List:
 
I have a text box on a sub form bound to a numeric field which contains,
obviously, a numeric quantity.  
 
If the user inputs a value in this box preceded by a '+' or '-' sign, I want
to add or subtract the value entered from the current value.  If they enter
a value without a preceding sign, I want to replace old the value with the
input value.  
 
I can obviously see if they entered a minus sign because the number is
negative.  But how can I know if they preceded the input value with a '+'?
 
MTIA
 
Rocky