I'm guessing... or I guess.....
A: How about over there? That place looks like a restaurant.
B: Yeah, but with all those servers in white tuxes, I'm guessing it's going to be expensive.

"I'm guessing...." sounds natural to me. And I feel like saying like that . But I always use "I guess..." only.
Would you tell me the difference between "I'm guessing " and "I guess ",please?

Question

I'm guessing...  or I guess.....
A:  How about over there?  That place looks like a restaurant.
B:  Yeah, but with all those servers in white tuxes, I'm guessing it's going to be expensive.

"I'm guessing...." sounds natural to me.  And I feel like saying like that . But I always use "I guess..." only. 
Would you tell me the difference between "I'm guessing " and "I guess ",please?

Answer

To me - there's very little difference. Being a continuous tense, 'guessing' can suggest a little more thought is going into it, but generally they are interchangeable in this sort of situation.

Answer

These are usually interchangeable.  The only example I can think of where they are not interchangeable would be this:

"Are you going to the game tonight?"
"I guess (so).  There's nothing else to do."

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!