Is this indoor or outdoor? Yellowing can be from either over/under watering. I worked at a greenhouse for a summer and we used a cheap-o little water meter for certain plants. To this day, I still use it - and it works perfectly to see where your plants are at; you could probably find this at Home Depot, Target, or a greenhouse. Or, order it on Amazon:
Amazon.com: Indoor/Outdoor Moisture Sensor Meter,...