Before sending any emails (especially a blast) it's highly recommended seeing what your email will look like in someone else's inbox.
There are two ways to do this:
Send a test email to yourself using the test button on the Email Composer dialog:
The test button will not show up until you have subject and email filled in. Once Test button is clicked it will show the test address that you can change to whatever you'd like.
IMPORTANT NOTE: If you use this method with 'Email All' button you will end up receiving a ton of mails - one for each Lead present in your search.
2. Add your email to the Lead and use the Send button.
This one is pretty straightforward. Send an email just like you would do it to any other lead - but in this case it comes to your inbox instead of theirs.
Using this option you can test out your Email Sequences as well - seeing if your conditions were set up right and if everything is working as intended. However, it won't work to test Reply Tracking if the recipient is the same email as your sending email.