If you take SWR measurements at a range of frequencies and plot them (or just watch the SWR reading as you change frequency), you should get a graph with one or more valleys of low SWR.
Ideally, if you find the ratio between the valley's actual position and where you want it to be, and trim or rebuild-bigger the antenna according to that ratio (or rather, the reciprocal of it — note that physical lengths go with wavelength, not frequency!), then the resulting antenna will be spot on.
In practice, it won't be exact because you're not building an exactly scaled antenna — your feed-point components and your wire diameter will be staying constant. But you can aim for a little bit too big, then trim it down until the SWR stops dropping.
The above procedure will get you minimum SWR for that antenna design. But remember that you can have a resonant antenna that still has the wrong resistance (i.e. not 50 Ω, assuming you're using normal equipment and coax). Resizing the antenna will not change the resistance — you have to change the shape or add a matching network.
The simplest way to figure this out is to use an antenna analyzer that can read out the resistance value. Find that minimum SWR point, then look at the resistance (or the magnitude of the impedance — which is the same here).
You can also tell from the shape of the SWR graph. A matched antenna will have a very sharp valley that hits 1:1 SWR. One with a different resistance will have a broader valley that does not ever reach 1:1.