Omitting letters and numbers.
Read words rather than phrases to locate letters and numbers that have been omitted. See the parts of the words. Look for "tricky" areas where something might be wrong. If you sense that something isn't right, look at it carefully.
Words or sentences that have been omitted.
Watch for entire words or sentences that may have been omitted.
Typing errors.
Proofread carefully to find simple typing errors, especially in short words.