Who gives us transition level when it varies?
ATC, ATIS, any others?
Check the country's AIP, ENR section.
ATIS, ATC or FIS is a good bet otherwise but it remains a bet.
In some EASA countries, you can calculate it yourself if the Transition altitude is fixed. E.g. Germany's TA is 5000ft, with the transition layer a minimum of 1000 ft. So with a QNH of 1014 the TL will be FL60, 978-1013 it'll be FL70 and 977- will be FL80. Others are different but they don't hide the information, it's in the AIP.