The iambic pentameter found in Act 3, Scene 1 is definitely tricky. However, one good example can be seen when the fight breaks out and Romeo begs Mercutio and Tybalt to stop fighting and also begs Benvolio to help him beat down their swords.
The iambic pentameter is especially tricky to spot in this passage because several of the lines are headless lines, meaning that they omit a first syllable, thereby not conforming to the typical meter. These lines are also tricky because they all have a hypercatalexis, meaning an extra, unexpected foot.
If we were to scan the first three lines, we would get:
__ Draw,' / Ben -vo' / -li -o'; / beat down' / their wea' / -pons __.
__ Gent' / -le -men', / for shame'! / for -bear' / this out' / -rage _!
Ty-balt', / Mer -cu' / -ti -o', / the Prince' / ex -press' / ly hath' (III.i.84-86)
We can see that the soft syllable is omitted in the first iamb in the first two lines, beginning with the syllables "Draw" and "Gent-." We can also see that a final stressed syllable is omitted in the final iamb, ending with the syllables "-pons" and "-rage." Not only that, we can see an extra iamb, or hypercatalexis, has been added for all three lines, giving us a total of six iambs instead of five. Hence, this is an example of iambic pentameter, but with a hypercatalexis.
The next line in this passage is more typical of iambic pentameter; however, it is formed with an elision, as we see from scanning it:
For -bid' / this ban' / -dy_-ing in' / Ver -on' / -a streets'. (87)
In this line, the syllable "-dying" from the word "bandying" is elided into one syllable so that "in" is the stressed syllable.
Finally, the last line in this passage is the most complicated to scan. The best guess is that the final line is both a headless line and also contains a pause that is being counted in the meter, giving us perfect pentameter, as seen below:
___ Hold', / Ty -balt!' / Good (_) / Mer -cu' / ti -o! (88)
As you can see, scanning Shakespeare can be a very tricky process as he does take very many liberties with the meter. What can help is figuring out what you absolutely know must be stressed and looking at the dictionary for stress marks can help a lot with that. Figuring out the rest can be a lot like fitting in pieces of a jigsaw puzzle.