How can I check what a file is before deleting it, without opening it?

Select the file in Finder and press the Space bar. Quick Look shows a preview — documents, images, PDFs and videos play right there — without launching an app. Press Space again to close it, or use the arrow keys to step through files while it's open.

It's the fastest way to review a list of big files: sort by size, then Quick Look each candidate and delete what you don't need. Get Info (Command-I) adds the size and dates — see what Get Info shows.