Feeding Tomatoes

With our newly covered polytunnel we are finally growing tomatoes seriously for the first time. I started off the comfrey tea last week and intend to use this instead of commercial tomato feed. Reading an old post, to find out when to start feeding, it says when the tomatoes start to look tired. Could someone explain, to an inexperienced tomato grower, what a tired tomato looks like please? 
We are growing Sweet Millions (x4) in a 4' x 4' raised bed which is filled with our own compost from goat, chicken and llama manure; it is then topped off with one 40l bag of peat free compost.
Any advice appreciated. 😊