According to the atlas, both species are found in Portugal although brunneum is much more patchy. There's also nitidinerve, and all 3 are pretty similar although I think we'd be able to see the colour in the wing if it was nitidinerve.
Further north you'd look at the thorax - Keeled skimmers here have dark thoraxes without the blue colour. In France I think this would be a reliable distinction (although someone may wish to correct me on that). But somewhere towards the south of Europe, not sure where, Keeled starts to have blue thoraxes like this one. The field guide wants me to look at the structure of cells in the wing, but we can't see them clearly. So the fall-back is the length of the pterostigma. According to Dijkstra, it's a lot longer in Keeled than Southern, pointing to Keeled as the ID for this one.
Usual disclaimer - as I've never seen Southern skimmer other than in photos, take all the above with a fair amount of salt.
Further to Swindon Addick's post, the key difference is the no. of double cells subtended by RSPL. In O. brunneum it is typically 3 to 5 but can be up to 7, whereas in O. coerulescens it is normally none. Unfortunately, this feature cannot be determined from the photo. The next feature to look for is the size and colour of the pterostigma, in O. brunneum it is small and brown and in O. coerulescens it is large and yellow which matches the photo. There are a couple of other differences, one of the main being the colour of the frons which is whitish with a blue tinge in O. brunneum and brown in O. coerulescens. Again, though not discernible from the photo!
When I'm in the field in southern spain surveying I am not usually in a position to see how many doubled cells there are below the rspl - but it is usally easy to see the white / brown face or the short orangey / long yellowey pterostigmas to separate Southern / Keeled.
In this photo I see long yellow pterostigmas and Keeled would go in my notebook - but if I got one or two wrong along a stretch of river it wouldn't matter too much!
I'm not usually one to encourage the use of habitat in identification but I find Keeled Skimmer to be in the areas with more (and denser) vegetation (as in the photo) and Southern Skimmer in the more open areas where it might be perched on single stalks with no surrounding vegetation - but that is just a pointer.
I have a photo of a UK Keeled Skimmer with 4 doubled cells below the rspl if it would be useful to confuse matters!