How can I tell if a MOSFET is enhancement-mode or depletion-mode?
Two things I want to add to the answers already given:


Don't trust the schematic symbol. You'll see the depletion-mode symbol used pretty often for an enhancement-mode part because it's easier to draw. (The symbols suggested on the manufacturer datasheets won't make this error, but some random application circuit schematic from the web is not trustworthy at all)
How to tell from the datasheet whether the part is enhancement mode or depletion mode. For an n-channel FET, if the \$V_{gs({\rm th})}\$ is greater than 0, then it's an enhancement mode device. If \$V_{gs({\rm th})} &lt; 0\$ it's a depletion mode device. For p-channel, it's the opposite: \$V_{gs({\rm th})} &lt; 0\$ means enhancement mode, \$V_{gs({\rm th})} &gt; 0\$ means depletion mode.