Both have an interest rate on the vertical axis, so I keep drawing the wrong one.
Accepted answer
Match the graph to what's changing.
**Money market**: nominal interest rate vs. quantity of money. Use it for monetary policy and money demand/supply — the Fed changing the money supply, or money demand shifting with the price level or real GDP.
**Loanable funds**: real interest rate vs. quantity of loanable funds. Use it for saving and investment — government borrowing, changes in national saving, capital inflows.
The cue is usually the actor: the Fed acting on the money supply points to the money market; government deficits or saving behavior point to loanable funds. Note the different vertical axes — nominal vs. real — since mislabeling that is an easy lost point.