You can use MATCH
for instance.
Select the column from the first cell, for example cell A2 to cell A100 and insert a conditional formatting, using 'New Rule...' and the option to conditional format based on a formula.
In the entry box, put:
=MATCH(A2, 'Sheet2'!A:A, 0)
Pick the desired formatting (change the font to red or fill the cell background, etc) and click OK.
MATCH
takes the value A2
from your data table, looks into 'Sheet2'!A:A
and if there's an exact match (that's why there's a 0
at the end), then it'll return the row number.
Note: Conditional formatting based on conditions from other sheets is available only on Excel 2010 onwards. If you're working on an earlier version, you might want to get the list of 'Don't check' in the same sheet.
EDIT: As per new information, you will have to use some reverse matching. Instead of the above formula, try:
=SUM(IFERROR(SEARCH('Sheet2'!$A$1:$A$44, A2),0))