How do I know if a charity is a registered charity?
That is easy – there is a list on the CRA website

How do I know if a charity is a good charity?
That is a much more complicated issue that we will try to discuss on this site and the answer can be very personal depending on your interests, experiences and values.

Should I give to a telemarketer calling on behalf of a charity?
In general, unless you have an existing relationship with a charity (such as your church, university, hospital etc) it is best not to donate on the phone to charities that are cold calling as in some cases 90% or more of the funds may be used to pay for the cost of the telemarketers. Therefore very little actually benefits the charity.