It's not just a guess. We know what elements are in the sun based upon the light emitted from it. Elements emit light at very specific wave lengths, and as such we can identify what something is based upon the light it gives off, or in some cases absorbs. Side note, that's how we first discovered helium, in a spectrograph of the sun.
As for science in general, nothing in it is a guess. Any theories we produce must be able to stand on there own without any hand waving.
EndGameOmega
It's not just a guess. We know what elements are in the sun based upon the light emitted from it. Elements emit light at very specific wave lengths, and as such we can identify what something is based upon the light it gives off, or in some cases absorbs. Side note, that's how we first discovered helium, in a spectrograph of the sun.
As for science in general, nothing in it is a guess. Any theories we produce must be able to stand on there own without any hand waving.