Slovenia still has border crossings with Italy and Austria, as it is not part of Schengen yet, but once you get off the main roads, you can simply walk across the border. Hikers do it all the time and there are even mountain huts right near the border catering to them -- no-one will check your passport there (although they have the right to do that). In effect, if you don't have a car, you can cross the border very easily if you know where to go. However, large groups of illegals are caught fairly frequently.
This, BTW, is what the point where Italy, Austria, and Slovenia meet:
The same holds true for the Slovenian-Croatian border, but in that case, the controls are likely to increase when Slovenia enters the Schengen area and that border becomes the EU's border as well.