smssplit
UI to explain how messages are split in SMS
Use here: https://chadselph.github.io/smssplit/
Caveat: Different sms gateways and different mobile operators across the world sometimes do things differently, this is just an example that can usually explain why your message got split. Hint: it's usually because of a punctuation mark or whitespace that isn't encodable in GSM.
Here's a Python API based on this algorithm.