My method has been to Google the stock, e.g. Siemens, finding "SI".
Then look up possible tickers to find a match
possibles = FinancialData["*:SI", "Lookup"]
Check the names and exchanges
FinancialData[#, "Name"] & /@ possibles
FinancialData[#, "Exchange"] & /@ possibles
And finally check the quote matches what I expect:
FinancialData["NYSE:SI"]