Utilize in SQL Monitor Analysis or Reports pages any SQL Server counter available in PERFMON.
For instance BRENT OZAR has an excellent method to diagnose memory pressure. The counter in use is SQLServer: Memory Manager – Memory Grants Pending which is not available.
If Memory Grants Pending is averaging over 0, queries can’t even start because they can’t get enough memory. You may have queries that have incorrect memory grants, or you may just desperately need more RAM.
Jessica Ramos commented
Happy to report that the Memory Grants Pending counter was added in v10.0.13.
Note: The perfmon counter isn't cumulative, so it's only captured when SQL Monitor samples when a memory grant is pending. (This particular sample happens fairly frequently at every 15 seconds.)
Red-Gate peeps, any idea if there is any traction on this request?