Ask them for the serial number (on the back of the foot or when they boot into OS X and click on the apple icon in the upper left corner and then on "about this Mac" -> "more info") and type the serial in the searchfield of appleserialnumberinfo.com (you only have 3 tries every 24h).
Other people use Mactracker, but I am too stupid to use it (I only installe dit once and searched a serial and it didn't give me an answer... and I had no will to figure it out any further).
An additional note on the PowerBook G4 1GHz, it is not the 12" nor the 17", but the 15" one! It is faster than the imac not because of the higher CPU only, but it also has 167MHz FSB and faster RAM (which will probably not do much), but also the best Graphics card. If I remember correctly Cubase isn't hard on the graphics card, but who knows). (you can see it all here http://www.everymac.com/ultimate-mac-comparison-chart/?compare=all-powerpc-macs you just have to pick each model from the list).
If you have several tracks under one another, the 17" iMac might be not bad, for working, the PowerBooks 15" display though will give you more "room" to look ahead in single tracks.
Maybe, if you are really cheap you want to consider all possible options (despite the towers, that you have no room for) and look up the prices on Ebay via their backwards search. If you search e.g. "PowerBook G4 1GHz and then hit enter, on the next page scroll down, till you see something like "sold items" on the very left. Mark that one and you will get all sold items of this kind and then you can sort it from price smallest to biggest (mind there might be defective ones in the lower price range, but you can look that up easily, I think).
Other people use Mactracker, but I am too stupid to use it (I only installe dit once and searched a serial and it didn't give me an answer... and I had no will to figure it out any further).
An additional note on the PowerBook G4 1GHz, it is not the 12" nor the 17", but the 15" one! It is faster than the imac not because of the higher CPU only, but it also has 167MHz FSB and faster RAM (which will probably not do much), but also the best Graphics card. If I remember correctly Cubase isn't hard on the graphics card, but who knows). (you can see it all here http://www.everymac.com/ultimate-mac-comparison-chart/?compare=all-powerpc-macs you just have to pick each model from the list).
If you have several tracks under one another, the 17" iMac might be not bad, for working, the PowerBooks 15" display though will give you more "room" to look ahead in single tracks.
Maybe, if you are really cheap you want to consider all possible options (despite the towers, that you have no room for) and look up the prices on Ebay via their backwards search. If you search e.g. "PowerBook G4 1GHz and then hit enter, on the next page scroll down, till you see something like "sold items" on the very left. Mark that one and you will get all sold items of this kind and then you can sort it from price smallest to biggest (mind there might be defective ones in the lower price range, but you can look that up easily, I think).