Check the sys.columns system catalog view:

SELECT *
FROM sys.columns
WHERE is_computed = 1

This gives you all computed columns in this database.

If you want those for just a single table, use this query:

SELECT *
FROM sys.columns
WHERE is_computed = 1
AND object_id = OBJECT_ID('YourTableName')

This works on SQL Server 2005 and up.

UPDATE: There's even a sys.computed_columns system catalog view which also contains the definition (expression) of the computed column - just in case that might be needed some time.

SELECT *
FROM sys.computed_columns
WHERE object_id = OBJECT_ID('YourTableName')

If you want to use the INFORMATION_SCHEMA views, then try

SELECT 
COLUMNPROPERTY(OBJECT_ID(TABLE_SCHEMA+'.'+TABLE_NAME),COLUMN_NAME,'IsComputed') 
    AS IS_COMPUTED,
*
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME='<Insert Your Table Name Here>'

For SQL Server 2000 the syntax is:

SELECT * FROM sys.columns
WHERE is_computed = 1

And the slightly more useful:

SELECT 
    sysobjects.name AS TableName, 
    syscolumns.name AS ColumnName
FROM syscolumns
    INNER JOIN sysobjects
    ON syscolumns.id = sysobjects.id
    AND sysobjects.xtype = 'U' --User Tables
WHERE syscolumns.iscomputed = 1

sample output:

TableName              ColumnName
=====================  ==========
BrinksShipmentDetails  Total
AdjustmentDetails      Total
SoftCountDropDetails   Total
CloserDetails          Total
OpenerDetails          Total
TransferDetails        Total

(6 row(s) affected)

If you have many tables with computed columns and want to see the table names as well:

SELECT sys.objects.name, sys.computed_columns.name
from sys.computed_columns 
inner join sys.objects on sys.objects.object_id = sys.computed_columns.object_id
order by sys.objects.name

Building upon Ian Boyd's answer you can also get the computed text (SQL query) and a sample select statement to test the calculated field using this query:

SELECT 
    sysobjects.name AS TableName, 
    syscolumns.name AS ColumnName,
    'SELECT ' + syscolumns.name + ' FROM ' + sysobjects.name as TestQuery,
    (SELECT TOP 1 definition FROM sys.computed_columns WHERE name = syscolumns.name) as Computed_Text
FROM syscolumns
INNER JOIN sysobjects ON syscolumns.id = sysobjects.id AND sysobjects.xtype = 'U' --User Tables
WHERE syscolumns.iscomputed = 1

Example: