If ou are using a SDR dongle with DSDplus then you could frequency calibrate that dongle to have it show the correct frequency.
Enter a known good frequency in SDR# or similar spectrum view program, a control channel are the best, then zoom in so that the modulation of the signal are covering maybe 1/4 of the spectrum and adjust the bandwidth, click and drag, so that it covers the signal exactly.
If it is outside of the signal at one end and way inside at the other end then use the ppm frequency correction in SDR#'s cogwheel settings to make the signal fit perfectly inside the modulation bandwidth. Then use that ppm value in DSDplus to get a correct frequency reading.
This is a perfect match:
View attachment 170546
/Ubbe