You can test forwarded port with a port scanner.
Just let it scan for port 27005 to 27015. You can not do this from your own lan/computer but has to be done from an outside connection.
(here you can find a port scanner)
However I almost know for sure that you need to open more ports for steam CS.
A quick search on google told me this:
ports to open:
tcp 6003, 7002
udp 27005, 27010, 27011, 27015, 27020