To answer the OP's questions;
- either by marking on the hollow cylindrical part (tube is typically manufactured to ASTM A269 for US tubing, or EN 10216-5 for European stuff).
- or by it's appearance. Tubing tends to be somewhat cleaner and shinier, if you know what I mean. Though (small bore SS316) piping can be shiny too, (SS316) tubing usually has a very bright appearance.