Let us remember that an iambic pentameter is a line of verse that has ten syllables and a regular rhythm that can be expressed as "de-dum de-dum" and so on with the emphasis falling on the "dum" or the second syllable. Actually, it is not difficult to find examples of iambic pentameter in Shakespeare's plays because the majority of the text is written in blank verse. There are some sections of each play that are written in prose, and these can easily be distinguished by the length of the lines and the fact that they are written in paragraphs, whereas blank verse is identified by the shortness of its lines in comparison. Here is an example from Act III scene 5 when Juliet says farewell to Romeo and has a vision of their future:
O God, I have an ill-divining soul!
Methinks I see thee, now thou art so low,
As one dead in the bottom of a tomb.
Either my eyesight fails, or thou look'st pale.
If you check each line, they all have ten syllables, making them excellent examples of lines that are written in iambic pentameter. There is some irregularity in the rhythm, but on the whole it is regular. Now have a look at Act III yourself and see if you can find some more examples. Good luck!
We can actually find an example of iambic pentameter in the very first lines of Act III. Benvolio, speaking to his good friend Mercutio, says,
I pray thee, good Mercutio, let’s retire.The day is hot; the Capulets, abroad;And if we meet we shall not ’scape a brawl,For now, these hot days, is the mad blood stirring. (3.1.1-4)
Bear hence this body and attend our will.Mercy but murders, pardoning those that kill. (3.1.157-158)