If you need to find out the type of RAM and it's speed or the number of slots available in your computer without having to open the case. Then you can do so in linux machines (Ubuntu or Fedora, etc) by using the following command:
?sudo dmidecode --type 17
It would generate output like this for each slot, so you can also count the number of slots:
# dmidecode 2.9
SMBIOS 2.4 present.
Handle 0x1100, DMI type 17, 27 bytes
Memory Device
Array Handle: 0x1000
Error Information Handle: Not Provided
Total Width: 64 bits
Data Width: 64 bits
Size: 2048 MB
Form Factor: DIMM
Set: None
Locator: DIMM_B
Bank Locator: Not Specified
Type: DDR
Type Detail: Synchronous
Speed: 667 MHz (1.5 ns)
Manufacturer: 7F7F7F7F7F020000
Serial Number: 00000000
Asset Tag: 000000
Part Number: PSD22G6672S