An EMB-140 First Officer reported runway hold short markings for Runway 15R at SPS are inadequate.

Narrative

Upon landing in SPS on 15C; we were instructed to hold short of 15R by Tower. After exiting the runway; I completed the after landing checklist; swapped frequencies; and contacted Ground Control. We were instructed to hold short of 15R. We assumed we passed it because the next markings we saw were white markings indicating the entrance of 15R. The Captain immediately informed ground control that we might have crossed the hold short line. Ground Control then cleared us to cross 15R and to the ramp. On our departure on 15R; we noticed a light aircraft approaching 15R on Foxtrot after landing on 15C. The light aircraft asked Ground if there was a hold short line for 15R because he did not see one. Ground control responded with 'you're fine right there'. The Captain noticed on our departure that there was in fact not a visible hold short line; only a CAT 2 critical area. I believe that this and future events could be avoided with proper signage and taxiway markings. We were obviously not the only aircraft that day that had problems identifying the hold short markings. I am also curious to find out how many times this same incident has occurred in the past at this airport.