I've literally said (in text or email to ex-MC), "You're still there, right?" And he's responded that he is. Or with current T, a couple times, if we've had a session where I revealed something big (whether about something from my past or regarding the therapeutic relationship), I've sent an email that basically just said, "You're still OK with everything, right?" And he's said he is. In terms of what response you get, I've found it's helpful to say exactly what you want. If you're literally just checking in to make sure they're still there/still care/whatever, just say that, even if it feels a bit silly. If you want support, say that (I've said, "I could really use some support right now" or "some words of support"). Or if you feel uncomfortable being so clear, then you could mention something stressful coming up. Or, I mean, if you literally just want to check in, if you don't have a set session time, you could just say, "I was just making sure I had the session time correct--is it 2 pm Tuesday?" (Doesn't work so well if you have a set time!)

But I agree with others that it could help just to ask your T what they'd want you to do in that situation, like OK to reach out with an "are you still there?" email? How would they respond to that? And to talk in general (during session) about feeling a lack of connection between sessions--maybe T would have suggestions for that (something like...dare I say, a transitional object, for example).