who is right when giving nutritional advice?

Generally, the information that stays along the same lines as what has been discovered before is correct. For example, take glucosamine. There are varying studies about what exactly it does in the body; however, the common answer of what it is, though, is an amino acid. All studies have shown it's an amino acid, and they all have shown that it helps with joints, to what extent though varies.

Get the example?

Nobody is 100% right or 100% wrong.

A good source of information is Wikipedia - any important studies are there. Another decent source are sites ending in .edu or .gov (although there are some exceptions).

Be skeptical of any claims without references to studies, or at least a good scientific explanation of WHY.

Ultimately you must make up your own mind based on reading a lot of different sources, and by using your common sense.

Stick to your 5 a day.

Recommended calorie intake, fat intake etc. (on many packets of food)

Don't regularly exceed 2-3 units of alcohol a day.

Pretty much it really.

The rest in the news is where they find possible links between foods and illness'. Which normally the risk is so small, you don't need to worry about.