These issues can sometimes be confusing, especially to newer collectors. Because the same stamp design is used for two different Scott catalog numbers, it's not always easy to know which one you have. Let's go over them in detail.
Scott #727 and #752. Both #727 and #752 were issued in uncut press sheets. Therefore, gutter pairs and blocks are possible on both issues. A mint copy of #727 always has gum. A mint copy of #752 has no gum as issued. If you have a copy with gum, it's definitely #727. If it has no gum, it can be either #752 or a #727 that someone soaked the gum off of it. For used stamps, there is no difference.
#733 and #753. A mint copy of #733 always has gum. The #753 has no gum. It's possible to manufacture a #753 out of a #733 by simply soaking off the gum. Collectors will collect #733 with gum. For #753, horizontal and vertical guide lines were added to the sheets of stamps. So for #753, collectors will obtain a copy that shows evidence of the guide line. Usually, they do this in pairs or blocks to show the guideline in between. But even with a single stamp, sometimes there are traces of the guideline left on the tips of the perforations that tells you that it is a #753. This is true for both mint and used copies of this issue. Always look for the guideline, or traces of it on the perforation tips for single stamps.
#730 and #766 (likewise for #731 and #767). Both catalog numbers were issued without gum. If you have any pairs or blocks of this issue with a gutter in between, it is definitely #766. The #730 sheets were already cut apart, so no gutter copies exist on that issue. If you have a single stamp, there is only one way to tell which issue it is. A normal margin on #730 is about 6 mm. wide from the edge of the stamp frame to the edge of the paper. If the single stamp has a huge margin on one or more sides, then that stamp must be #766. How much is huge? There could be slight variations in the margin size of #730 as the souvenir sheets were cut apart. I'd use 12 mm. (twice the normal size of the margin) and call that a huge margin copy. If you have a single stamp with 4 normal margins, there is absolutely no way to tell if it's #730 or #766. It is possible to take a full sheet of #766 and cut it down to individual souvenir sheets that match the size of a normal #730. I doubt that has happened much, if at all.
#735 and #768. The same reasoning applies as that with #730 and #766. If you have a gutter pair or gutter block or a single stamp with extraordinarily huge margins, it must be #768. If it is a single stamp with normal margins, it is impossible to tell which catalog number it is. The normal margin on #735 is about 8 mm. I would use 16 mm. (twice the normal size again) as the minimum for a huge margin copy. I think it's safe to assume that any full souvenir sheet is #735 and not a cut down version of #768.
#751 and #769 (likewise for #750 and #770). If a pane or single stamp has gum, it would be #751. If you have a pair or block of stamps with a gutter in between, it must be #769. The only way to know for sure with a single copy is if it has an extraordinarily huge margin, then it must be #769. But with normal margins, you can't tell a single copy of #751 and #769 apart. The normal margin on #751 and #769 is about 12 mm. wide. I would go with 20 mm. or larger to say that it is a huge margin copy.
For the rest of the Farley issues, the difference is obvious. For the National Parks issue (#756-769), they come imperforate only. Likewise for #754, #755, and #771. The imperforate copies all have a higher catalog value than their perforated counterparts. It's unlikely that someone would take an imperforate Farley issue and reperforate it because it would reduce the catalog value. Someone may have added perforations as a gimmick. Or someone may have done this to use up a huge lot of Farley issues as discount postage (perforating lots of sheets at once may be easier than cutting them up with scissors). Someone may take a perforated copy and trim off the perforations to make it look like the higher catalog value imperforate copy. Like any US issue that exists imperforate, these are usually collected as pairs or blocks to show that the item is fully imperforate. That way, you can be certain it's an imperforate issue and not a single stamp with perforations that were trimmed off. Any single imperforate stamp with very narrow margins should always be treated with suspicion.
Many of the Farley issues have no gum as issued. Used copies are almost always favor cancellations. Someone would have to take time and add glue or some other type of adhesive to the back of the stamp before affixing it to a letter for mailing. I'm sure a few Farley issues actually went through the mail this way. But the majority of the used copies though are what we might call "cancelled to order" because someone postmarked them and they never actually went through the mail. That's why most used copies have nice, light cancellations because someone took the time to cancel them and made sure that the cancellation didn't obliterate the stamp design.
In closing, I should also note that someone may take a copy of a stamp that has no gum as issued and regum it to make it look like its gummed counterpart. This could be true for #727 and #752, for example. It's important to check for original gum on those issues that were issued with gum.