The only real way is to have it "phone home." When it does, it will update the status of the subscription. I've gotten some TiVos from eBay that initially look like they had subscriptions, but when they phone home, the sub status will change, invariably to "8: Account Closed."
So, you should simply attach it to the phone line or install an network adapter, and have it phone home. Then check the status in the System Information screen. If it is a lifetime sub, call TiVo and have it transferred to your account. If it is a monthly sub, just use it as is. There is no way to transfer the sub to you.
Another way to test if it has a sub is to go to www.tivo.com
and attempt to subscribe the TiVo. If it already has a sub, you will be immediately informed that it has a sub by a red text message saying "Oops, the TSN you entered is already subscribed..."
Regarding the OTA question. Yes, it will work OTA until the digital transition in June. In order for a Series 2 TiVo (a 540 is a Series 2) to work with OTA signals, you must have a Digital to Analog (DTA) converter box attached to the TiVo. The software version of the TiVo also has to be at least 9.2, so you'll have to have the TiVo phone home and download the latest software before it can control the DTA converter box.