(Cripes, the above messed up on the input for some reason, re-read this one it will make more sense :)

Ok. There are questions you are asking, which I think are a litte invalid.

First of all, yes there is a way. I'll try my damdest to explain it in laymans terms, but no promises :)

Okay, let's quote some of the previous;

murray; << For example if I see a piece of music with no key signatures >>

If you are looking at a bare music score (with no guitar chord pictures above the stave), it WILL indicate the key signature. If there are no flats or sharps indicated, it is quite simply in C. If a melody was in the key of F# and there were no sharps or flats indicated, it is not written correctly. (I don't know how advanced your music theory is, but believe me, the above is correct).

Now we have that out of the way let's move on.

You said, << I was wondering if there is a similar way to tell a tune is in a pentatonic scale >>

Well, instead of asking that question, try asking this one instead. << I was wondering if there is a similar way to tell if a tune UTILIZES a pentatonic scale >> - A more valid question.

A melody isn't written in a scale, it's written in a key. That key will utilize scales for it's melody, and this is where the clarity of your initial question becomes hazy. The melody utilizes a scale and is not written in a scale.

Now we are edging closer to our answer.

Given the fact the our key signiture is x, we can look at the notes within. Let's say that x were to = C. If the flat third, major fourth and flat seventh were exclusively featured, we could say that the melody is utilizing the pentatonic minor scale. However, if the melody utilized the above notes PLUS the augmented 4th (flat 5th), then we are no longer utilizing the pentatonic minor, we are utilizing the blues scale. The difference being, we have the extra note that differentiates the blues scale from the pentatonic minor.

There are many different scales, and the initial note can often be the factor that determines which scale you are actually playing, but no matter what the key, the scale will often change throughout the piece, and only experience of recognition will tell you what scale you should be playing at any particular part of the melody.

Given that music is all too often predictable, it's fair to say that if a melody utilizes a particular scale initially, it will utilize that same scale ultimately. :-)

I'm hoping you follow this, I will add to it if there's any part you don't follow.