Pin poin a particular variable error in sql 2008 script

Pin poin a particular variable error in sql 2008 script

I have big sql query like this:
Select Distinct [Student].[Class].roll_nbr as
[PERIOD-NBR],[Student].[Class].ent_nbr as [CLASS-NBR],IsNull(Stuff((SELECT
CAST(', ' AS Varchar(MAX)) + CAST([Student].[Subject].ent_nbr AS
Varchar(MAX)) FROM [Student].[Subject] WHERE [Student].[Subject].roll_nbr
= [Student].[Class].roll_nbr and ([Student].[Subject].class_nbr =
[Student].[Class].roll_assignment_nbr or
([Student].[Class].roll_assignment_nbr = '0' and
[Student].[Subject].class_nbr = [Student].[School].bus_stop) ) AND
[Student].[Subject].ent_nbr <> '' FOR XML PATH ('')), 1, 2, ''), '') AS
[OLD-STUDENT-NBR.OLD],IsNull(Stuff((SELECT CAST(', ' AS Varchar(MAX)) +
....
It goes on and on and a page long query. Which builds a report. Problem i
am having is some variable is erring out with message:
Error converting data type varchar to numeric.
This is very genaric error does not tell me which variable. Is there any
way to pin point which variable is erring out in sql 2008?