Building on @cioddi's answer, as you can pass parameters to the Handlebars helpers, you could make it a generic function so that you can easily retrieve any value dynamically, e.g.
Template.registerHelper('session',function(input){
return Session.get(input);
});
You can then call it in your template like this
{{session "isCurrentUser"}}
Note that the auth packages come with a global helper named CurrentUser that you can use to detect if the user is logged in:
{{#if currentUser}}
...
{{/if}}
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