If a project on sourceforge does not specify any license, can I assume (L)GPL or no license (completely free)?
Yes, by all means assume that copyright doesn't hold and then watch yourself get dragged through the courts when you find out you were mistaken :-)
The default state for software in signatories to the Berne convention is that works are protected by copyright. In the absence of an explicit licence to state otherwise (by the copyright owner, not some hosting site), you should assume you have no extra rights.
I would suggest contacting the copyright holders and asking them to clarify the situation.
Depends on the country the source originates from and the country you are in.
In most European countries there is a copyright on every written text (including source code), even if it isn't mentioned explicitly.
Most companies I worked for handle "code without any license" as "closed source" and "do not use".
Try to contact the original authors and get a package with a suitable license.
And remember: I am not a laywer!
No, you cannot assume a license. And no license is completely different from totally free (this sentence sounds like a recipe to get sued).
But sourceforge hosts only open-source-software. So somewhere the licence should be specified. Can you say which project do you mean? Possibly we can then find out the license.