The MTU is 1500 by default unless you change it, this is nothing that the Check Point software has to deal with. Do remember that the MTU is just the total packet size and had nothing to do with the MSS which is the actual value you want to know, the value that shows how much data a packet can transfer, this is shown in the SYN and SYN-ACK packet and can be altered by setting up MSS clamping.
When you want a full picture abiout MTU, MSS path MTU Discovery and MSS Clamping read this article:
When you want to know how to deal with it in Check Point see SK 101219.
For testing a specific connection for the MSS use a free program called TCPoptimizer and use the MTU/Latency page to find the largest MTU for the connection you want to know. The MSS value is calculated by taking the Largest MTU and substracting 40 from it for the IP and TCP headers.