In As You Like It, Act III, Sc. 5, is Rosalind's monologue "And why, I pray you" written in verse or in prose, and how can you tell the difference?
In As You Like It, Act III, scene v, Rosalind's monologue is written in Blank Verse, also know as unrhymed iambic pentameter, which is what Shakespeare used most frequently in his plays.
Whereas prose does not follow a specific beat pattern, blank verse typically has five iambs per line. Each iamb is made up of two syllables, with the accent on the second syllable. You can see how this works in the monologue below, where I have marked the accented syllables in bold:
I pray you, do not fall in love with me,
For I am falser than vows made in wine:
Besides, I like you not. If you will know my house,
'Tis at the tuft of olives here hard by.
Will you go, sister? Shepherd, ply her hard.
Come, sister. Shepherdess, look on him better,
And be not proud: though all the world could see,
None could be so abused in sight as he.
Come, to our flock.
When spoken, the acccented syllable typically gets more emphasis, though not always in a truly pronounced way (as in the second line above, where the word "than" would not normally be emphasized strongly).
Also, blank verse is not always perfect - often, there is one leftover, unaccented syllable at the end of the line (as in the sixth line above). Less frequently, there can be two extra syllables, as in the third line above; this line cannot truly be said to follow the requirements of blank verse.