Something I wish more riders and dog owners did is regularly feel down their animal’s back, so they know what it normally feels like. Once you know your animal’s “normal”, you’ll be able to pick up on any new tension if and when it appears.
It’s also good practice to regularly watch your animal being led up in walk and trot in a straight line. It isn’t always easy to spot differences if you haven’t done this before, but with practice you can learn to look out for changes in movement. Stand behind your animal as they’re led away from you, then in front as they’re led back towards you.