If your razor isn't sharp, it isn't going to shave. It should be as sharp or at least nearly as sharp as a decent double edged safety razor blade. Just calling it a straight razor does not imbue it with magical anti-whisker super-powers. If you expect the razor to shave when it is not sharp, you are engaging in faith based shaving. This is the irrational belief, in the absence of evidence or logic, that the razor is capable of shaving your face.
Believe it or not, most razors do not come sharp from the factory. To a beginner, a fresh from the box razor might feel sharp, compared to his pocketknife, and maybe he can scrape a few hairs off his forearm with it, but it is not sharp. Some vendors hone the razors that they sell. Some do not. Vintage razors and Chinese budget razors are often sold online, with claims of shave-readiness. Internet shave ready and true shave ready are not even close to the same. Shave Ready is the new buzzword that sellers feel compelled to tack on to their item descriptions. But most of those sellers actually know nothing about straight razors, not even how to shave with one, let alone how to hone it or pronounce it shave-ready. What I am getting at here is that depending on who you buy a razor from, you may be getting a razor that is too dull for shaving your face.
The most authoritative means of determining whether or not a razor is capable of shaving or not obviously is to shave with it, or at least try to do so. Unfortunately a beginner does not have that option. He does not yet posess the requisite level of skill and familiarity with the razor to judge the sharpness of the edge. He does not know how much responsibility for poor shave performance is his fault and how much is the razor's fault. He might think that his shave is perfectly okay when he is actually suffering greatly and wearing a patchwork of stubble fields on his face. And so there are tests that remove some of the subjectivity from evaluating the edge.
One popular way of evaluating a razor's sharpness is called the HHT, or Hanging Hair Test, see this link: http://www.coticule.be/hanging-hair-test.html. The HHT is still somewhat subjective, but if you don't get a HHT3 or better then it should be no surprise to you if the razor does not shave well. If you do not even get HHT-1, it should be a surprise if it DOES shave. Results vary depending on exactly how you administer the test, particularly the hair sample used. It is a good idea to use hair that comes in packs from a beauty shop. That gives you several years' supply of testing hair with a texture that is sort of normalized among a large number of shavers. But no matter how careful you are to follow standard procedure with an average hair for a sample, HHT results for the exact same razor can differ from one tester to another. And yet a razor that is not giving you any indication of cutting ability is probably not going to have any cutting ability, and trying to shave with it is just wasting your time and tormenting your skin. Also, after you have tested a few razors and shaved with them, you get a pretty good baseline for your testing, and future tests on the same razor or another razor will have some validation, for you and you alone.
Another test, one that I like, is the Treetop test, or TTT. To perform this test, simply sweep the razor above the forearm, not touching the skin with it. It should pass about 1/4" above the skin. If the edge is pretty good, it will sever the tips from at least a couple of hairs. At the very minimum, it should barely treetop at 1/8" above the skin. If it will not do that, then it is not worth trying to shave with.
In an effort to normalize TTT results with the usual referenced HHT results, I have assigned arbitrary values to the TTT results, as follow:
Less than TTT-2 is simply unacceptable. Again, the hair sample and your technique can influence the results, but generally these will hold true for most folks. TTT-3 or TTT-4 are pretty good. You will get a decent shave from such an edge. TTT-5 will seem miraculous, or even mythical, if you have never experienced such an edge. TTT-5 is right up there with a very good DE blade. You will not get such an edge from common ordinary honing and even if the honer knows what he is doing and has the right gear to work with, TTT-5 on an edge will be cause to celebrate. This edge will have so much cutting power that you will need to lower your shave angle considerably from normal, to prevent cuts or excessive exfoliation. At TTT-2 you will need to use a fairly wide shave angle. Normal is with a gap between the razor's spine and the face equal to the thickness of the spine. A large angle will be about one and a half times the spine thickness. Any further out than that, and you are scraping, not shaving. A half spine thickness would be considered a tight shave angle, appropriate for the sharpest of straight razor edges, or maybe a shavette with a half DE blade in it.
There are other sharpness tests, usually used in the honing process, such as TNT, or ThumbNail Test, or the TPT, or Thumb Pad Test. You can look them up. They aren't really of much use when evaluating a completed edge for shave-worthiness.