Weighing them is really the easiest and safest way. Like Laurago said their weight should be somewhere between 90g to 110g the most. Like humans, "ideal" weights can vary- one way to visually tell if a tiel might be starting to put on weight is looking at their chest. An underweight tiel will have a very visible bone in the middle of their chest; overweighed tiels sometimes have a line down their lower belly. If your tiel is overweight it's sometimes cause by an all seed diet. Add pellets along with seed and offer a variety of veggies everyday. Overweighed tiels are very prone to weakened immune systems and tumors so it's important to weight your tiel on a regular basis.