You know what I'd do? Buy a digital kitchen scale! Just a cheap one from Wal-Mart or whatever, for like $10. It's hard to say what underweight is, especially with no pictures. But for yourself, a scale may come in very handy. I have one and use it all the time for my snakes. It's good to see how they grow and change, and if there were any feeding issues to see if they lose weight. I also log in snake notebooks for mine (feeding days, sheds, changes in behaviour, etc.).
All the best!
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