Explain how you could determine wheater seeds in packets of round pea seeds have a genotype rr or rr

I think you mean Round could be RR or Rr (round is dominant to wrinkled)
R=round
r=wrinkled
RR would be round phenotypically
Rr would be round phenotypically
To determine if they are RR or Rr you should perform a backcross with a wrinkled mutant!
I put a picture to demonstrate this.
If they are RR then if you cross with rr you would only get phenotypically Round pea plants.
If they are Rr then if you cross with rr you would get 50% round and 50% wrinkled. Any wrinkled? Definitely the heterozygote Rr!!