The A4 sized suspension files are just a few millimetres larger than A4. This saves space but means that pages do need to be aligned to stop the edges hanging out the edge of your file.
If you have a file handy, measure the width of the cardboard on the file. If it measures to 360mm (14.1 inches) then you are using foolscap. A measurement of 305mm (12 inches) would be an A4 size suspension file.
Without a file available, you can measure the distance between the rails. A foolscap rail distance will be about 390-400mm. For A4, the distance will be around 340-350mm.
Some desk pedestals allow either Foolscap or A4, however they do this by allowing the Foolscap one way in the drawer and A4 at a 90-degree angle. Although you have a choice which size of file to use, you can only use one or the other since once you have fitted one file in one size, the other size will no longer fit.