To identify the rhyme scheme, it is necessary to consider Keats' entire poem. It features 5 stanzas, each with ten lines. Each stanza develops its own rhyme scheme that follows the same pattern.
The first four lines of each stanza create an abab pattern of rhyme, while the final six lines of each stanza follow the pattern of cdecde with occasional variation, such as those found in the last six lines of the first two stanzas.
The line you mention is found in the final stanza of the poem:
O Attic shape! Fair attitude! with brede
Of marble men and maidens overwrought
With forest branches and the trodden weed;
Thou, silent form, cost tease us out of thought
As doth eternity: Cold Pastoral!
When old age shall this generation waste,
Thou shalt remain, in midst of other woe
Than ours, a friend to man, to whom thou
"Beauty is Truth, Truth Beauty," --that is all
Ye know on earth, and all ye need to know.
The first four lines here follow the abab rhyme scheme (brede/weed rhyme, as do overwrought/thought).
The final six lines follow the basic cdecde rhyme scheme but with a minor variation: cdedce. (This is the same variation in rhyme scheme found in the last six lines of the first stanza.) Pastoral/all, waste/sayst, and woe/know are the rhymes in this last stanza. Woe and know make a perfect rhyme, while the other two are approximate rhymes.
Do you mean the meter of this verse instead? You need more lines of the poem before you can determine the rhyme scheme, which involves listening to the sound of the last syllable in each line. For example, look at the following:
Slowly, silently, now the moon
Walks the night in her silver shoon,* *shoes
This way and that, she peers and sees
Silver fruit upon silver trees.
("Silver" by Walter de la Mare)
The sounds of the last syllable in each line (moon, shoon, sees, trees) indicate a rhyme scheme of aabb, also known as rhyming couplets. You simply assign the letter a to the first sound, then b to the next sound, and so on to determine the rhyme scheme or pattern of rhyme.
Meter, on the other hand, is determined by listening to the stressed and unstressed syllables in a line and seeing if there is a pattern. Your line could be read as follows with the capitalized syllables being the stressed or accented ones.
when OLD age SHALL this GENerAtion WASTE
A pattern emerges of an unstressed syllable followed by a stressed syllable; in fact, there are five units, or feet, of this pattern, which we call iambic. We can identify the meter of this verse then as iambic pentameter, meaning five units of a stressed-unstressed pattern.