like the body language of someone who planning to break an agreement? 

" The principle of good faith is a rule of customary international law... A dispute about compliance with this obligation could be raised under the applicable dispute resolution mechanism ...  It is likely that there would only be a finding of breach where it was supported by clear evidence." para 14 of yesterday's summary of the Legal Position on the Withdrawal Agreement 

"Article 184 ... requires the EU and the UK to use their “best endeavours, in good faith” to negotiate the agreements governing the future relationship “expeditiously” ... This is an obligation as to the conduct of the parties, rather than an obligation to achieve a particular result"   para 77 

"... the provisions in this article are subject to the dispute resolution procedure  ... including reference ... to an arbitration panel whose decision is binding. An arbitration panel would be able to consider, for instance, whether the parties had acted in good faith  ...  A tribunal would only find a breach of the duty of good faith if there was a clear basis for doing so... In the absence of such a clear basis, a panel would be unlikely to find that a party had acted unlawfully in reaching its view under Article 20 about whether the Protocol continued to be necessary."   para 11 of the Annex.