I assume you are talking about the radial velocity method.
Analysis of a radial velocity curve yields the minimum mass of a planet $M \sin i$, where $M$ is the true planetary mass and $i$ is its orbital inclination (90 degrees would mean the orbital plane is in our line of sight).
In general, with no other information that's all you can say. In principle $M$ could have any value $\geq M\sin i$.
Of course increasingly small values of $\sin i$ are increasingly unlikely and the average $\sin i$ of a randomly oriented population is $\pi/4$.
If you see a transit then you know that $i$ is close to 90 degrees and can in fact estimate $i$ from the transit duration.
If you measure the star's rotation period and can estimate it's projected equatorial velocity from spectral line broadening, then you can estimate it's inclination to the line of sight. One could then assume that the planet orbits in the equatorial plane of the star and use the same inclination for its orbit.