Its a matter of the objective intention of the parties. You have to look at the circumstances to see what is intended and weigh up the different factors.
If the parties agree that they will hold as a JT or TIC, that will be determinative.
If the parties are in a long-term relationship or have children, its more likely to be a JT.
If there are unequal contributions to the purchase price, its more likely to be a TIC.
If its students living together, its more likely to be a TIC.
There is a slight presumption in favour of JTs but its a fairly weak presumption that you can displace with evidence that a TIC was intended.