how check bitcoind default values How can I see which values bitcoind is using for eg. dbcache, or other memory and connection params ? If the value is NOT set in bitcoin.conf. Is there a bitcoin-cli command to check all the params ?
You can see all available command options by running the command bitcoind -?. 
Similarly, in the GUI you can check out "HELP" -> "Command Line Options" in the task bar. 

This will return a list of all available commands, as well as their default values (when applicable). Note that this just returns all possible command options, not specifics about what you have set. Default values will be used unless you specify otherwise, and I believe the hierarchy for specifying a command option is:

An option passed on command line when starting bitcoind 
(takes precedence over)
an option passed in .conf file 
(takes precedence over)
an option set in the GUI 

(Information pulled from here)