Learning how much compost is actually useful in an established bed

My vegetable bed has had compost added every spring for years. The soil is dark, drains reasonably well and grows healthy plants, yet I still follow the old habit of adding another thick layer because garden advice says ‘more organic matter.’ Is there a point where that stops helping or creates nutrient imbalance? I'm thinking of a soil test before adding much more. What signs tell you an established bed needs compost rather than just mulch and careful watering?