A couple of rules apply when proofing or having your web site proofread.
1. Check your site over more than once.
2. Have someone else read your content.
3. Always check your own material even if you hire a proofreader.
4. If you're going to hire a proofreader, hire someone that has a proven record or is in the business. Don't hire your web designer's wife.
You should look for the following:
1. Incorrect spellings
2. Missing punctuation
3. Missing words
4. Check the tense
5. Formatting errors (extra spaces, missing spaces, etc.)
6. Style problems (Don't switch around your style. Stay true to your design.)
Run through your content for each of the above separately. It will make finding an error easier.
Colleen Degnan Johnson