Generally we like to see that you have put effort into trying to solve your problem. It looks like you have been trying things here: http://www.mathworks.com/matlabcentral/answers/32252-field-text-number and that this is not exactly an overlapping question ...
I start by defining a function iscomma
function iscomma(src, h)
if strfind(get(src, 'string'), ',')
set(h, 'string', 'error');
else
set(h, 'string', 'ok');
end
Then I create two uicontrols and set the callback of the edit box to iscomma
h1 = uicontrol('style', 'edit');
h2 = uicontrol('style', 'text', 'units', 'normalized', 'position', [0.5, 0.5, 0.5, 0.5]);
set(h1, 'Callback', @(src, evt)iscomma(src, h2))
