I have data in an unknown projection to me.

I open it in QGIS and I see that the shapes are ok, but I need to identify the projection in order to merge this data with other stuff.

The points look like WGS84 (EPSG:4326) but without comma.

If a point in WGS84 is like this: (-58,-34)

In this projection the coordinates are something like: (5670000,6120000)

Any ideas?