There is almost nothing better in the world of food than a perfectly ripe tomato, eaten warm from the sun. Knowing when you have a ripe tomato ready for picking can be the difference between heaven and a mulchy mess. Particularly with heirloom tomatoes, which can be different colors (even striped and speckled) it can be hard to tell by looking if your tomato is ripe.
The first way to check if your tomato is ripe is to check if the skin has turned from a dull, matte surface to one that is glossy and slightly shiny.
Next check the color, it should be fairly deep (unless you see on your tomato plant tag or seed packet that it isn't).
Your ripe tomato will give slightly to the touch - it shouldn't be soft, just a little tender.
Pick and enjoy!