Look at the media part of the GUI, it will show how many GB there has been written to each cartridge. There is no "remaining capacity" field as the data compression can be very variable and therefor impossible to predict. But a rule of thumb should be tape "capacity * 1.5".
LTO2 : 200*1.5 = 350GB
LTO3 : 400*1.5 = 600GB
LTO4 : 800*1.5 = 1200GB
You may get more you may get less. The media list will also show if tapes are full, partial full or empty (scratch).