Can you help me identify the word that should be capitalized in this sentence? George washington' protrait is on the one dollar bill.

There is no word in this sentence should be spelled out fully in capitals, but there are words where the first letter of the word should be capitalised. 

Proper nouns (such as names of places or people) should always start with a capital.

For example, you were writing an account of a disagreement you had with a friend over whose portrait was on the one dollar bill, and your friend that it was Abraham Lincoln, you might write it as: "No, Abraham Lincoln is on the five dollar bill. George Washington's portrait is on the ONE dollar bill."

Letters That Can Be Capitalized

In your sentence above, an extra letter needs to be changed a capital letter, the 'w' in 'washington', as the first letter in people's names become capitals, as in: George Washington or Tim Cook.

Other cases when the first letter of a word is changed to its capital include: The first letter of a new sentence; the days of the week and months of the year; the names of languages and nationalities; proper names (people's names, names of countries or geographical areas); names of institutions; acronyms; significant words in titles of works of art; and so on.

This films explains further on when to use capital letters: