If you flush the toilet and water comes up it to your tub, or in to the basement drain that is a good sign you have a blocked drain. You may try to empty the tub and find soapy water coming into your toilet. This can also indicate you have a clogged drain. When there is an obstruction the water will come back up the drain; when it finds another route the water will go there. The water or sewage will seek the lowest place that it can access as well as the largest pipe. Any combination of water leaving one fixture and entering another, even in different rooms or on different floors is usually an indicator of a clog somewhere in the line.