To find out if you are Joint Tenants, you will need to check on your Title Register Document. If you have a mortgage then this will be held by your mortgage company, but for £4 you can now check your Title Register Document online in the "Public" section of the Land Registry website at https://www.gov.uk/government/organisations/land-registry.
The Title Register Document will show the names of the people that own the property and, if you are tenants in common will also have wording similar to: "No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the court".
If there is no such wording in the Title Register Document then you are almost certainly joint tenants.
Quick Tip: When you've got your Title Register Document, download it and print it out: it also tells you your Land Registry Administration Area and your property's Title Number - both of which are required for severing the Joint Tenancy.