harmonize melody

 If I'm harmonizing a melody in major scale and in the middle of the melody it switch to minor relative. how I will know that it change to minor relative?. 

Thank you

I think that would be something that'd be easier to figure out by looking at the bass and the overall harmony, or even just listening to whether the 'character' of the mode sounds major or minor. 

However, in Classical harmony, if the melody has moved from a major scale to its relative melodic minor scale, you'd probably be able to tell by looking at the melody alone from the addition of a new leading tone that leads into the new minor root note. So, if you were in C major and you started seeing lots of G-sharps (and F-sharps) in the melody, they may be there because you've switched to the relative melodic minor of A. 

I think the best way to tell if you've moved to the relative minor would be to determine whether there was a perfect cadence ("V --> i") into the minor key. In our C major / A minor example, this would mean an E major chord (E-G#-B) leading into the new "i" chord, A (A-C-E). 

In my experience, Classical composers tend to shift between the two pretty freely. It's fairly easy to get into the minor mode by a perfect cadence and use another perfect cadence to return to the major key. You could have something like this:

I - IV - I'6/5 - V - I - IV - V - III - vi

(Here we switch to the minor key: 'vi' becomes 'i.')

i - iv - i'6/5 - V - i - V - i - V/III - III

(Just as before, this is where 'III' would become 'I' again.)

[ At the end, the term V/III just refers to 'the fifth of III' in your minor key. In the C major / A minor example, going from V/III to III in the key of A minor is simply G major to C major. ]