After executing the following command get the correct result
./check_nrpe -H @IP -p 5666 -c alias_mem
OK: committed = 6.083GB, physical = 1.608GB|'committed'=6.08327GB;12.7978;14.39752;0;15.99725....
The issue is that I haven't found any documentation that lists all the possible commands.. I've tried alias_cpu, alias_disk and it works but I'd like to find all the possible commands ...