How can I check if isp blocks port 25 for incoming traffic?

I want to run a mail server and experience problems receiving mail. After googling around I found a potential reason to my problems. My isp might be blocking port 25 for incoming traffic.

After calling them a couple of times the support techs have given me different answers. One of them told me that port 25 was completely blocked and the next one that only outgoing traffic (unless through the isp server) was being blocked. I tried calling them a third time and that tech wasn't sure....

Now I wonder. Is there an objective way for me to figure it out myself. I want to be 100% sure about the answer before I try other mail server configurations because I don't like this maybe or maybe not dilemma.