how to get all merged cells at a time

Is it possible to get all merged cells by some kind of filter in excel.

What do you mean how to "get" them? You mean find them?

Microsoft support for finding all merged cells

Click Home > Find & Select > Find.

Click Options > Format.

Find dialog box
Click Alignment > Merge cells > OK.

Check the Merge box
Click Find All to see a list of all merged cells in your worksheet.

By the way, merging cells is almost always a bad idea. If you want to unmerge all the merged cells you can just select the sheet and if Merge & Center is shaded, click it to unmerge all merged cells. You don't have to identify which ones they are.