Distance = Rate * Time
This is a good time to re-enforce practical applications of the Distance / Rate / Time equations.
You should know the the speed of your plane, and your desired pattern size, you can describe the distance in terms of time.
Example: If you're in a C-172, at a pattern speed of about 90 kts, you're making about 1.5NM per minute (or about 1.7 statue miles per minute). If you want your downwind to be 1/2 mile away, parallel to the runway, that is about 17 seconds away.
Use some rough estimation, that you're still making distance in your turns, then you probably want your crosswind and base legs to be about 15 seconds.
I think you'll find that someone who cannot judge distance accurately can still estimate where they'll be in 15 seconds. Ask the student to count out loud to 15 on their cross/base legs, and decide if they are too close or too far.
Also, talk through the implications if they're going faster or slower; if they have a tailwind/headwind. Get them to actually pencil out the math, or make it an E6B exercise.