I would ask. You could just say, "What's your policy on outside contact?" Or "I thought about texting you the other day, but wasn't sure if it was allowed?" For my T, he allows emails (and will always respond), but texting is only for scheduling. But it varies by T, from not allowing any outside contact besides for scheduling, to allowing some forms or within certain time frames, to allowing any form (email, text, phone call) at any time (my ex-marriage counselor was like this, but didn't necessarily respond right away).