The lack of a FX mix knob tells you it's a serial loop...
The difference:
A serial effects loop puts things after the preamp in a series connection. 100% of the signal flows from the preamp into whatever is in the loop, then back in to the power amp. This is the ideal type of loop for use with most effects and MFX units. However, it is critical that high quality effects are used as every bit of your precious signal is going through these units. Tone suckage may occur if cheap effects are used.
A parallel FX loop has "parallel" signal flow (signals running 'side by side'). In a parallel FX loop the signal comes out of the preamp and is split
one signal line (let's call this line A) runs throught the stuff connected in the FX loop, just like a serial connection
The other line (line B) goes from the preamp right to the power amp. The FX mix knob mixes line A with line B, but only up to 90%. You can never have a 100% wet signal with a parallel loop.