What is the best way to make sure I get my W2's from my past employer when past employer doesn't have my current address on record?

Contact you past empolyer and give them the current address. If you have moved and not let them know I am not sure this is something that would get forwarded.

The USPS, as a matter of policy, does NOT forward tax documents; the information contained in such documents is too sensitive.

THAT is why it is important to make sure your employer has your correct address.

When you call the employer, ask if they will scan-and-email or fax the W-2 to you. As long as the W-2 copy is accurate, clear and legible, the IRS will accept it with your tax return.