A rental should be able to sharpen them if you address it. (And at least here in Europe all rentals sharpen their skis frequently enough).
A good tester is to see if the skis edge grips the surface of your finger nail, the same way you would test a knife or a sharp tool (https://www.youtube.com/watch?v=hHEuV2CPE3w). If it slides across it with no friction, then it's a blunt ski.
Personally, I would advice against trying to tune skis manually, because you might just as easily blunt the edge as you might sharpen it if you don't have a good jig and tools.
Through my 12 years of teaching skiing, I will say that more often than not people who complain about icy conditions are held back by their technique; are you skid steering or carving, because when you are skid steering no level of ski tuning can make your edges stay on hard snow. (Tip: This guy has made an incredible amount of videos about the topic and has even himself progressed from being a skid steering freestyle skier to now being an excellent carver. https://www.youtube.com/watch?v=lwWFlltWQTU).
A fair point; my technique is... slightly less than stellar. Especially when I hit ice, I tend to panic and try to slow down however possible. So I'll pay attention to sharpness next time I rent, but blaming the equipment is a favorite pastime of mine (read: golf).
A good tester is to see if the skis edge grips the surface of your finger nail, the same way you would test a knife or a sharp tool (https://www.youtube.com/watch?v=hHEuV2CPE3w). If it slides across it with no friction, then it's a blunt ski.
Personally, I would advice against trying to tune skis manually, because you might just as easily blunt the edge as you might sharpen it if you don't have a good jig and tools.
Through my 12 years of teaching skiing, I will say that more often than not people who complain about icy conditions are held back by their technique; are you skid steering or carving, because when you are skid steering no level of ski tuning can make your edges stay on hard snow. (Tip: This guy has made an incredible amount of videos about the topic and has even himself progressed from being a skid steering freestyle skier to now being an excellent carver. https://www.youtube.com/watch?v=lwWFlltWQTU).