Consider looking at the following:
page life expectancy - (I think 5 mins is the MS best practice) this will tell you how long data pages are living in memory
buffer cache hit ratio - (I think 99+% is the MS best practice) this will tell you how frequently SQL goes to memory for data
pagefile reads - (lower the better) this will tell you if the system is using the page file and reading (memory issue)
disk queue reads / writes - this will tell you if the box is having trouble fetching data from disks assuming it's not in the cache
To me, memory is something that should be configured according to the duties of the box. If, say, the box is solely for SQL, I'd recommend using the majority (90%) for the database engine. Note: this will probably set off alarms (SCOM or other alert software) because it'll look like the box is slammed, but it really isn't. It can also result in crazy-high page life expectancies which isn't a bad thing, but can indicate that the database doesn't need that much memory and can possibly take on more work. I hope that kind of makes sense.