You can use the UPDATING() function here which expects the column name as string whereas UPDATE() expects an identifier.

For an example, see the second FAQ I cited in my answer.

I try "IF UPDATING(wCol) THEN", but I have an -143 Syntax error near 'wCol'. Here wCol is a long nvarchar.

Hm, in my tests with 17.0.9.4857, I could avoid the syntax error but within a cursor loop, the UPDATE search condition does not seem to work properly.

Here's my attempt with a trigger on the sample database's Products table, with several debug messages within:

create or replace trigger TU_Products
   after update on GROUPO.Products
   referencing old as o new as n
for each row 
begin
   message 'Trigger "TU_Products" fired' to log;
   for lpp as crs cursor
      for select column_name as @ColName
          from SYS.SYSTABCOL SC key join SYS.SYSTAB
          where creator = user_id('GROUPO') and table_name = 'Products'
          order by column_id
      for read only
   do
      message 'Trigger "TU_Products" checks whether column "' || @ColName || '" was modified.' to log;
      if update(@ColName) then
         message 'Trigger "TU_Products": Column "' || @ColName || '" was modified.' to log;
      end if;
   end for;
   if update(Quantity) then
      message 'Trigger "TU_Products": Column "' || 'Quantity' || '" was modified.' to log;
   end if;
   message 'Trigger "TU_Products" finished' to log;
end;

-- Update one row
update Products
set Quantity = Quantity - 1, UnitPrice = UnitPrice + 0.50
where ID = 300;
-- display the last 11 log messages
select msg_text from sa_server_messages(null, -11) order by msg_id;

The test shows the following output:

Trigger "TU_Products" fired
Trigger "TU_Products" checks whether column "ID" was modified.
Trigger "TU_Products" checks whether column "Name" was modified.
Trigger "TU_Products" checks whether column "Description" was modified.
Trigger "TU_Products" checks whether column "Size" was modified.
Trigger "TU_Products" checks whether column "Color" was modified.
Trigger "TU_Products" checks whether column "Quantity" was modified.
Trigger "TU_Products" checks whether column "UnitPrice" was modified.
Trigger "TU_Products" checks whether column "Photo" was modified.
Trigger "TU_Products": Column "Quantity" was modified.
Trigger "TU_Products" finished

Apparently, the "if update(@ColName)" test did not succeed for the "Quantity" column whereas the test via "if update(Quantity)" does - as does the test via the alternate syntax with "if updating('Quantity')".