Nokia has published WP7.8 software update availability table for all the regions. It shows the latest version of software available for your phone. To check for your phone’s current software version, go to Settings > About > more information > Firmware revision number.
Your phone notifies you once an update is available. We ask you to be patient, as updates for Nokia Lumia range are always released gradually, and it can take up to three weeks for the update to become available for everyone. To make sure you don’t miss out on the notification, check on your phone that under Settings > Phone Update > “Notify me when new updates are found” is enabled.
Click here for checking your WP7.8 Firmware number.