There is a check for this pair that works even on a bad phone line, and it does not require hearing the difference at all: try to stretch the sound.
Betacism, and Why It Has a Name
Spanish treats b and v as effectively the same sound, a merger common enough to have its own name in linguistics: betacism. Korean has no /v/ at all. Both routes leave you with one sound where English keeps two, which is why this pair can feel genuinely inaudible at first rather than merely difficult. Any check that sidesteps the missing category and asks a question your ear can already answer will beat straining to hear a contrast you have no slot for.
Lips Together, or Teeth on Lip
For b, both lips press shut, pressure builds behind them, and the sound is the release — a small burst. For v, your top teeth rest lightly on your bottom lip and the air keeps flowing through the gap the entire time. The mouth shapes are genuinely different, which is good news: this is not a subtle distinction once you know where to point your attention.
The Reliable Test: Try to Stretch It
A v can be held. You can sit on it for as long as your breath lasts, buzzing, like a small engine. A b physically cannot be held — the moment the lips open, the sound is finished. That gives you a check that doesn't depend on hearing the difference at all: ask whether the sound had any length to it. If something continued, it was v. If it was over instantly, it was b. This works even on a bad phone connection, where the finer detail gets lost.
When the Sentence Won't Help You
Say either of these out loud and the mouth shapes are obvious. At speed, in a room with other noise, they are not:
In a meeting
I need your boat.
I need your vote.
Both are grammatical, both make sense depending on the day. When two options fit equally well, your ear has to do the work alone — no context arrives to help.