Detecting and confirming sentiments in chat messages
You need to define best.

If best means being exactly sure of your labeling, then you'd have to manually go through all messages, use crowdsourcing and what not. This is assuming that the chat messages are in a language that you can understand.

If best means automating the process, you could use an existing sentiment analysis model to predict the sentiment of each message and see if it lines up with the existing labeling. Of course, perfect alignment is unlikely.

If best means doing it from scratch, then I would try to cluster the sentences into 3 groups and then look at the composition of the groups. Of course, this is highly dependent on the clustering and how you define the distances between the messages