[CentOS] centos 5.5 check memoray usage too high???

We have DELL R900 server with 128GB RAM (CENTOS 5.5)in it.  This server only have one application running and few people use it.

Every week I ata least get one or two messages from monitor tool mail to me say:

Message=Memory Utilization is 92.02%, crossed warning (80) or critical (90) threshold. 

Since server have 128 GB RAM and only 1 application.  I really don't belive that.  Does there has some way can check memory utilitation ?

Thanks.