Is RFLP markers are dominant or codominant?
I suspect that you "know" the answer to your question and are just seeking confirmation.  First, let's get some terms straight.  Dominant and recessive are used in reference to phenotypes, so one needs to have a transmissible phenotype to score among the offspring of our genetic cross.  Phenotype are usually due to (or caused by) mutations.  In a diploid species, if you can only score the phenotype in offspring that are homozygous for that mutation (denoted m/m) then the phenotype is recessive to the wild-type allele of the gene.  In other words, a heterozygous offspring, (m/+) will have a wild-type phenotype.  On the other hand, if a heterozygous (m/+) offspring has a mutant phenotype then we would say that the mutation is dominant over the wild-type allele.
Your question is asking about a test cross of a +/+ individual to a homozygous mutant (m/m), however, in this case the RFLP does not typically have any mutant phenotype, it is just a convenient genetic marker for the purposes of this discussion.  All of the cross-progeny will be (m/+), and if you go to the trouble of preparing genomic DNA from those F1 cross-progeny and perform a Southern blot with a probe that detects the RFLP, then you will be able to demonstrate this.  Typically, unless the RFLP-bearing chromosome is homozygous, the probe will also detect the wild-type restriction fragment, so there will be two bands on the film, one for each allele.  Since you can detect the genetic marker in the F1 generation, the RFLP is dominant, but since you can also "score" the wild-type fragment, the two fragments are indeed co-dominant (after you do the Southern blot).