The only exact way to find out is to remove the lid and take a look inside.
There are 16 sockets on the mainboard (please refer to the file attachements below: Picture1_MainRAM.jpg). These 16 sockets are for the "Mainboard RAM" and are arranged in 4 rows each with one second. All four chips in a row must be mounted.
All 16 chips will be installed from the factory if your unit runs software version 28.xx or higher (or has been correctly upgraded to 28.xx or higher).
Older TC2290 might have a ME00 Memory Expansion board installed as well. (Please refer to the file attachements below: Picture2_ME00.jpg).
This memory expansion board offered to have additionally 28 seconds of RAM arranged in 7 rows of 4 seconds. Again, all four chips in a row must be mounted. Please note: we have not been able to offer this option since 1996.
The value in SPEC# 7 must be set to the number of seconds installed: one second for each row of the mainboard RAM + four seconds for each row fitted on the ME00 board.
Please also refer to this FAQ that explains the versions and expansion options.