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})} < 0\$ it's a depletion mode device. For p-channel, it's the opposite: \$V_{gs({\rm th})} < 0\$ means enhancement mode, \$V_{gs({\rm th})} > 0\$ means depletion mode.
In most supplier's search engines (and in manufacturing selection webpages) the enhanced-mode MOSFETS are considered as "normal" or "standard" and may not have any clear label as "enhanced", while the depletion-mode transistors are explicitly designated as "depletion mode". Here is an example from Digi-Key:
AFAIK, there is no common convention on how the depletion-mode transistors are designated, all is vendor-specific.
Rather than look at symbols you need to examine the device datasheet.
An Enhancement mode FET such as your IRF3710 will have a couple of VGS = 0 characteristics with a Vds current that is very low, usually this is the Vds breakdown voltage or Vds leakage current specification. There will also be a VGs(threshold) specification, which is the beginning of conduction (starting to turn the FET on):
For a depletion mode FET such as the DN2625 the same leakage current specifications will show a non-zero value for VGS (the device must be turned off to measure the breakdown or leakage currents). The inverse of the VGS(threshold) to turn on the enhancement mode FET is the VGS(threshold) required to turn off the depletion mode device (be careful here, in that the value is that required to reduce ID to the leakage current, not the threshold to begin lowering the current):
