Since there is no “automated” way to establish the key in Cubase you have to do it by inspecting the music.
Assuming the piece is diatonic, if you look at how the song ends you will almost certainly find a cadence of some kind - usually, but not always, a perfect cadence. This is a V I chord sequence (ie dominant to tonic). If the cadence is present then the last chord will (almost certainly) be the tonic of the key of the song. For example if you find G7 C as the last two chords then there is a high probability the song is in C. Or more accurately, it ends in C - it may have started in some other key and modulated to C before it ended. If you don’t find a perfect cadence look for the plagal version which is IV I (ie subdominant to tonic). If the piece is in C and finishes with a plagal cadence you’ll find F C chords at the end. There is slight complication with songs in minor keys which sometimes finish with a major chord; for example a piece in Am might finish on an A chord.
Once you’ve established the key the song ends in, you can go back and look at the beginning of the song. If there is an introduction it may very well lead into the “main melody” from the dominant (the V chord) - another clue to the key (especially if it confirms what you discovered at the final cadence). The subdominant chord is also a clue here. If you find a preponderance of, say, F chords among Gs and Cs then this is quite good evidence this section of the piece is in C. D, G and A chords point towards D and so on.
However, even if you’ve discovered how it starts and ends, there may be key changes somewhere inside the song and it’s worth looking for these by finding imperfect cadences and seeing where they are modulating to. For example, a passage in a song that you’ve established starts in G (possibly because you found D7 G cadences) kicks off with a sequence like D7 F7 Bb followed by further cadences of F7 Bb and a lot of Eb chords is very likely to have modulated to Bb.
Basically all you need is a bit of music theory and some detective work to establish a key.