Usually a call to the airline's FF program will get you the info you require. Generally, they require a copy of your FF card and most recent statement mailed or faxed to them to verify you claim of elite status in your current airline.

If you are comped, you will usually find that it is for the balance of the current year and the next full year only. After that you have to quailify as usual. But be certain you are planning to use this benefit to it's max, because most airlines only allow this "match" opportunity once in a lifetime.