"Did sir just call me Dobby?" squeaked the elf curiously from between its fingers. Its voice was higher even than Dobby's had been, a teeny, quivering squeak of a voice, and Harry suspected -- though it was very hard to tell with a house-elf -- that this one might just be female.
"Tell with" is not a set phrase defined in the dictionaries I've looked up. So, how should we understand the "tell with" in this context? Is it a common set phrase?