Things ended badly — how you know you are in a dispute
At the risk of sounding trite, it is not hard to work out when you are in a dispute.
You will generally be the accused or accuser of an allegation that you or another have not honoured certain of the terms of the contract.
A prudent contractual party will generally notify their concerns in writing.
You will know of their unhappiness from the letter if you are blamed. It will identify, possibly in general terms, the contractual term which you are bound by and set out your failure to honour or perform that term.
If you are accusing the breach, you will know that your expectations have not been met and therefore there is a problem that will need to be fixed one way or another.