how can I see a password session variable ?

From what I've seen, the only real way to do this is by copying the secure value to a non-secure variable in a VPE branch rule. See this question (and possibly this one if you're running 11.4) for more info.

Basically, you'll need to get the password value in a VPE branch rule and set it to a normal variable. It seems you can't get secure variables from within an iRule or other places besides during the policy workflow. Taken from Kevin's response on the first question I referenced, you may do something like this:

session.custom.pass = return [mcget -secure {session.logon.last.password}]

Hope this helps.