Sorry about the jargon, Maria. I didn't know how much you knew.

Use a form to input your data, not a table.
Open the form in Design View.
Right Click on the Check Number control (text box)
Go to Properties
Click on the Event tab
Click next to AfterUpdate, choose Event Procedure, click just right of that
and open up a code page.

Just above where it says

End Sub

put the code

Dim LastNum as Long
LastNum = NZ(DMax("[Check Number]","TblOfCheckNumbers"),0)
If Me.[Check Number] > LastNum + 1 Then
Me.[Check Number].Undo
MsgBox "That was not the next check number. The previous check number was "
& LastNum
End If

Replace TblOfCheckNumbers with the real name of the table that contains the
check number.

I'm assuming above that you don't want the code to automatically fill in the
next check number but if you do, open an 'After Update' code page for a
different control in your form, one you always fill in when you are entering
a new record (perhaps the date?) and enter:

Dim LastNum as Long
LastNum = NZ(DMax("[Check Number]","TblOfCheckNumbers"),0) + 1
If Me.[Check Number] =LastNum
End If

Put things in the After Update event of a control means that when you have
finished entering something into a text box, or choosing something with a
combo box, your code will happen.

One problem you will have though, if you really do have a field in your
table called Date, then before you do much else, you really ought to rename
it.

Look up Access Reserved Words field names to find out what may happen if you
don't. Call it XDate or anything else (and be prepared to spend time mending
queries, reports and forms)
You can always change labels in forms and reports to say Date so long as the
field doesn't

Evi