All you said is correct,but you would like to run single chain gel,you have to denature the DNA and run it in formaldehyde gel.
If you would like to know whether the DNA is single stranded or double strand,run a regular gel and a denature gel with the DNA,you would be able to tell.
If the DNA is single stranded 750 base,in the regular gel it will move faster than 750 bp,but will be 750 base in denature gel.