I'm not too sure how you'd get them to lose weight, other then cut back on anything fatting they eat ( seeds,etc)
But I do know that www.drsfosterandsmith.com sells scales for birds I've seen it in every magazine I get ( it normally has a budgie sitting on it for the picture) but I never paid attention to the price
I read one way to tell is to feel the pectoral muscles; if they bulge away from the center keel bone, then the bird is probably overweight I find the best way to tell is by weighing them, average weight is usally between 90-110 grams, Georige was getting a little bit chunky I just cut back a little on the seeds no millet or treats and gave her more pellets and fresh vegetables I got my scale from walmart and it was under 20.00
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.