There are various error-checking functions available in Excel, and most of them would be appropriate for your situation. The simplest would be to use IFERROR and just refer to a cell on one of the sheets, like this:
=IFERROR('2020'!A1,"Missing")
If the formula receives an error value because it can't find the cell, it will display the text "Missing"; otherwise, it will show the value of the cell.
The disadvantage of doing it this way, hard-coding the sheet name into the function, is that if the sheet is deleted the sheet name will be replaced in the formula with #Ref!, so even if the sheet is restored the formula will need to be changed.
We can enhance the functionality by using an INDIRECT along with the IFERROR. This will take a text string and turn it into a cell reference. So assuming we have worksheet names in column A, we could do this:
=IFERROR(Indirect(A1 & "!A1"),"Missing")
The INDIRECT function takes the value in A1 and combines it with the text string, and reads the whole thing as a cell reference. This has the same result, but is more robust since deleting the sheet and then replacing it will clear the error.
For a slightly more elaborate result, we could next IF and ISERROR instead of using IFERROR. It gives us more control over the result.
=IF(ISERROR(INDIRECT(A1 & "!A1")),"Missing","Found")
ISERROR returns TRUE if the first argument throws an error, an FALSE if it does not. This will return "Found" if the sheet is present, or "Missing" if it isn't.
