Typically, you’d use the Get-Member cmdlet:
$someVariable | Get-Member # or: Get-Member -InputObject $someVariable
Those two examples are subtly different, if $someVariable happens to be a collection. The first one will pipe the objects in the collection to Get-Member (so you’ll see the properties and methods of the items in the collection); the second example will show you the properties and methods of the collection object itself.
For most types of objects, you can also look them up on MSDN. Use the following command to get the type name, and just plug it into any search engine. The MSDN documentation will usually be right at the top of the list:
$someVariable.GetType().FullName