[Steenrod]

How can I (using a chemical test)  distinguish between  CH_3CN  and CH_3NC?Thank you.

[discodermolide]

How can I (using a chemical test)  distinguish between  CH_3CN  and CH_3NC?Thank you.

You have methyl isocyanate, look up its reactions. The other is a solvent acetonitrile.

[Babcock_Hall]

The rules of the forum are that you must show an attempt.

[mamid]

You have methyl isocyanate, ...

Actually, it's methyl isocyanide or methyl isonitrile. Otherwise, discodermolide is right: there's a huge difference in reactivity between the two, the nitrile being much less reactive than the isonitrile.
Another difference is in their smell. While acetonitrile/methyl cyanide smells like a tannery, i.e., not particularly pleasant, methyl isonitrile beats it by a huge margin, both in power and unpleasantness.

[discodermolide]

I apologise for my spelling mistake.
I would not advocate smelling methyl isonitrile, it is a very dangerous compound.