Before we embark on the (admittedly daunting) key to the species of Festuca, it’s a good idea for you to get a thoroughly good idea of what a fescue looks like. The most sensible species to use is Festuca rubra: it’s in flower now, it’s very abundant, and it’s easy to identify. 
Go out into your nearest grassland and ignore the conspicuous Holcus lanatus, Dactylis and Arrhenatherum. Look at ground level for the grass with the narrowest, most hair-like, bright green leaves. Pull up a handful big enough to guarantee having a flowering stem in it.
Shake off all the other grasses and you should be left with a flowering individual of Festuca rubra (left). It is a good idea to check that it’s not Deschampsia flexuosa (it should have straight, not wavy (right), panicle branches).
The panicle should look like this (left) and the spikelets like this (right). Measure both of them to the nearest mm. My panicle was 75mm long and my spikelet 10mm long (excluding the awns).
Look (X10) at the ligule (left) and the top of the leaf sheath (right). Notice that the leaf sheath is closed at the top, and there are no shoots emerging from it other than the culm (the flower stem).
At the bottom of the stem, note that the non-flowering branches (tillers) all emerge from the base of the plant, rather than from within the sheaths of older tillers.
The important thing is to practice dissecting the spikelet and measuring the sizes of all the component parts: the two glumes separately, a central lemma (body and awn separately) and the palea inside it.
Here are the measurements I got from my plant: lower glume (the smaller of the two) 3.5mm, upper glume 5.0mm. Lemma 6.3mm and its awn 2.1mm. Palea 5.6mm, anthers 3.3mm, ovary (hairless) 0.7mm, anthers 3.3mm and stigmas 2.2mm.
Finally, look at the underground parts and see if there are any rhizomes. Yes, there are. The whitish, fat root sticking out to the right is what a Festuca rubra rhizome looks like.
So there you have it. That's all there is to it. Once you have had a couple of days to practice, we can embark of the key to the 13 species of Festuca in Stace. Look on the bright side. There are 170 species of Festuca in Flora Europaea.
The hair-like leaves are folded, and without unfolding them, mine measured between 0.7mm and 1.0mm across.
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