There are quite a few examples to choose from in this memorable poem. I will pick out some of the similes from the beginning of the poem and hopefully this will enable you to see how it is done so that you can find some more in the rest of the poem. When we think about similes we are always talking about literary comparisons, when the author asserts a comparison between one thing and another, that normally we would never associate with the first object it is being compared to. However, good similes make us see the comparison and see how they are relevant. We can identify similes because they use the words "as" or "like" in their comparisons.
The first example of a simile describes the skylark's upward motion:
Higher still and higher
From the earth thou springest
Like a cloud of fire;
The comparison therefore compares the skylark's movement into the air to a cloud of fire shooting up, emphasising the skylark's vitality and quickness.
Secondly, in the third stanza, the motions of the skylark are compared:
In the golden lightning
Of the sunken sun,
O'er which clouds are bright'ning,
Thou dost float and run;
Like an unbodied joy whose race is just begun.
Comparing the skylark to an "unbodied joy" expresses something again of the energy of the skylark and its pure joy of living.
Hopefully this will help you identify more similes that are used in this poem. Good luck!