'How do you know you have a familiar?'
It's obvious, the familiar will contact you or you contact them, sometimes they float around waiting for you to notice, other times not so much - some can theoritically 'slap you across the face and tell you to look up'. Keep yourself open to it and watch for signs that a Spirit is trying to get in contact with you.
There are various ways to tell if a Spirit is trying to contact with you - and it's not always a simple little 'Hey, Listen!' If they know you can't 'hear them' they will try an assortment of other means.
Signs - leaving signs in your path, can be so obscure or can be down right simple that something is trying to contact you
Divining - you can use an assortment of Divination practices to find out if a spirit is trying to contact you - you can then further use this method to talk to them until you can hear them properly. (This is a method I use from time to time)
Dreams - even though this can be a little 'untrustworthy' because of our lovely subconcious mind, sometimes a Spirit will use your dreams to speak to you - why? because your mind is a lot more susceptible to 'spiritual events' whilst you are in a sleeping/dreaming state; you're also susceptible to events happening outside of you - such as if someone started carrying you while you were asleep, you could dream of flying (Many people have confirmed this - that in their dream they started flying for no reason - probably because their body was aware that it was being moved)
Obtaining and or Finding a Familiar
You can 'find' your familiar or look for one, there are a large number of spirits out there who would be willing to form a working relationship with you, some spirits also 'enjoy' being in a subservient position, though, that does not mean every spirit likes serving - think of it like people, so people get more joy out of serving people than lording over someone.
Different practitioners see it differently and that is okay, just as long as you keep in mind that a Familiar is a SPIRIT and not a PET , though that doesn't stop the idea that your deceased pets can become your familiar.
That different beliefs of 'obtaining a familiar' are as follows
They Find you
You beseech some deity or 'higher' spirit to send you one
A mentor passes theirs a long to you, or they will find one for you
A 'Coven' would give you either a group familiar or the beings they worship could present you with one
Spirit Journeying/ Reaching out to the 'great beyond' in hopes of finding one.
Ritual Creation - some witches believe that a Familiar can be created - or at least confuse the ideas of Fetch and Familiar together. (Despite their heavy similarities) this would probably fall along the lines of an egregore or thoughtform but is still treated as a legitimate spirit.
None of these methods are garunteed, nor is it guaranteed that you will ever 'obtain' a familiar - because some just might outright refuse you, the end.
What do you do when you have one?
Most practitioners set a vessel on their Altar, this is a contained vessel like a small jar or maybe a bowl in some cases, this vessel generally has some 'homely' touches since the Spirit will be making your home as their own, also this vessel would contain something to represent the Spirit - like a talisman, charm, herb, animal part etc.
Traditionally: Practitioners would never leave their 'Familiars' in plain sight, generally they would tuck them away in either a Chimney, or disguse them on the Mantle (Hearth) but these days, it is easier to have a working space out in the 'open' in your home.
Much like working with any other spirit, you need to give them offerings, food etc so that they stay 'healthy' and will happily work for you - as well as regular cleaning - such as removing old offerings from the Altar or workspace and replacing them with new ones - cleaning their vessel and placing what you took out back in. (The Spirit will inform you of what Vessel they would like - if they want one etc talk to it, find our what it likes)
Some practitioners like to 'open' the vessel (Literally if its a Jar or Bottle) and let the Familiar out for 'exercise' as in some spirits do not like being crapped up all day and like to 'return' to whatever it is they came from for some much needed rejuvenation - if you're relationship is good and healthy they will return after some time - the time however, is it specific (Oldly, much like you would do with a Pet - though they are NOT PETS - just a simple analogy)
The Black Toad - Gemma Gary
Traditional Witchcraft: Book of Cornish Ways, Gemma Gary
Treading the Mill - Nigel G Pearson
Deed Without a Name - Lee Morgan
Wicked Enchantments: a history of the Pendle Witches and their Magic - Joyce Froome
MrShadowbwoy - Youtube