I hope you don't mind getting a bit geeky.
*.ipa files are just zip files. To look inside you can:
1. You can copy the files to a new location.
2. Rename the copies to *.zip.
3. Unzip the files.
4. Use the finder to look through the unzipped folders.
I believe that Apple requires correct icons for each device so you should be able to tell by looking for the icon files in the package.
I used Temple Run 2 for this example. After expanding the zip archive I had a folder named
Temple Run 2 1.0 copy.zip Folder in my safe location. The icon files are located in
Temple Run 2 1.0 copy.zip Folder/Payload/templerun2.app/Icon.png
You can tell if it is universal by looking for the "Icon*.png" files.
Icon.png - iPhone
Icon@2x.png - iPhone Retina
Icon-72.png - iPad
Icon-144.png - iPad Retina