I've found reading work out loud (sometimes recording it and playing it back) is a great way of checking to see if my dialogue is up to scratch. If I stumble over my words, a reader will too as out with the old editing pen again! It is wise to use accented speech sparingly. You want to give a sense of what a character's accent is. You don't need to use an accent for each and every word they say. Again, that is tiring to read, especially in a longer work.
You need your characters to speak in different styles so readers can easily tell them apart during "conversational pieces". Sometimes this can be done by the choice of words a character uses. Sometimes it can be that Character A always speaks in short, sharp sentences, while Character B takes their time in getting to the point!
I love getting my characters to "chat" even if sometimes it is to themselves via their own internal thoughts. This is where you and, later, a reader can find out so much about them.