How do you find out what transmission is in an 88 suburban?

You have one of these two

Suburban - 1/2 ton 1987-1992 TH700-R4/4L60

Suburban - 3/4 ton 1987-1990 TH400/3L80

Wish to confirm see link below.

Hope This Helps.

Check your steering coloumn. If it is factory and does NOT have the overdrive(the D outlined) then you probably have the TH400. You can also tell by the shape of the transmission pan and sometimes the pan has either a code or the actual "TH400" or "TH700-R4" stamped on it. Get underneath with a cameraphone and take a pic and go to the auto parts store or dealership.