One of the biggest problems with growing tomatoes in containers is having the soil drying out and the water no longer being absorbed. The biggest sign is wilting tomato plants that don't recover after watering. It the pot completely dry out the soil becomes Hydrophobic, just like the desert, and the water will run down the sides just wet the top and bottom, leaving the roots in the middle bone dry!
You can check by digging into the side of the pot or, if you plant is small enough you can carefully pull it out of the pot.
If the plant is still dry in the middle it will take several repeated watering on the same day to get the soil moist again.
A water catch tray under the pot will also help rehydrate the soil by allowing water to pool and allow the soil to suck the water up.
A water catch tray can also help keep the plant from drying out in the first place.
I've even taking smaller pots and submerge them in a large bucket of water.
Another sign that your plant has dried out is if you pick up the pot and it is unusually light.
Hope this helps in your quest to grow the perfect tomato!
Chef / Farmer Greg