Does it have characters who fall in love with each other? Then it might be a romance, but is not necessarily. The "fall" is significant: romances are about the deepening of a relationship. If the characters start off happily in love and continue to be so without the relationship becoming more intense/fulfilling, it's not a romance. The characters can start off in love, or start off married, as long as there are still major obstacles to overcome in the relationship.
Is the majority of the book devoted to interaction between the romantic leads and/or the romantic leads' thoughts about each other? Again, not necessarily a romance, but might be. If, on the other hand, you have a romantic subplot between two characters who spend 60% of the story occupied with other characters, it's almost certainly not a romance. The Hunger Games, for example, has a romantic subplot but for most of the book Katniss isn't neither thinking about nor interacting with her love interests.
Is the main plot of the story about the relationship between the romantic leads, or is it about something else? The interaction between the leads of a romance does not have to be romantic in nature throughout the entire story: they can start off fighting each other, or as partners solving a mystery, or whathaveyou. But the focus does need to be on how the characters interact -- on their relationship and how it changes over the course of the story -- and not how they interact with other people/how they solve the mystery/how they prevent global war/etc. This is subtle but very important. You can have two different books about spies who are trying to stop a dangerous weapon from falling into enemy hands and who fall in love in the process, and one might be a romance and the other a thriller. It all depends on what the focus is on. If the romance is tacked onto a gripping narrative revolving around the whereabouts of a stolen deadly virus, then it's a thriller. If the stolen deadly virus is a MacGuffin that serves as an excuse to put the spies in close proximity to each other so they can fall in love, it's a romance.
Is it mostly about sex? A romance can be completely chaste or it can be riddled with sex scenes. However, if the book is mostly "the leads having lots of sex without much thought or development in their relationship", then it's probably erotica rather than romance. This is a gray area -- if it's 2/3rds or more sex, I'd say it's almost certainly erotica. If it's less than 1/3rd, it might be romance. If it's between 1/3rd and 2/3rds, it might not be satisfying to readers looking for erotica or romance.
Do the romantic leads spend a significant portion of the book reflecting on their feelings? This is neither necessary nor sufficient, but it's still a good indicator. If a book rarely shows either of the romantic leads thinking about their feelings for one another, it's probably not going to be a very good romance even if it otherwise falls into the genre.
Does the setting/worldbuilding provide excuses to make the feelings of the romantic leads more intense? This, too, is neither necessary nor sufficient, but is an indicator. If the novel is about werewolves who soulbond for all eternity with their one true love, it might be a romance.
Does the narrative include in the denouement a scene where the romantic leads come together to joyfully discuss the resolution of the problems in their relationship? Another neither-necessary-nor-sufficient, but if the story doesn't have this kind of payoff -- the "awww" moment where they talk about their feelings for one another and the misunderstandings or whatever that have caused them problems up until now -- it's probably not a romance.
Does the story* end with the romantic leads in a happy, committed relationship with one another? If it doesn't, it doesn't belong in the romance genre. Readers may well describe it as "romantic" when a love story ends in tragedy, but this is not what romance readers are looking for, and they will lynch you if you market a tragedy to them as part of the romance genre. No, really, they will hate you forever. Just don't.
* In most cases "story" = "novel", but in theory you can have a romance-genre series with one love story that spans multiple novels. This used to be more common, but it's not nearly as popular with romance readers as having the love story wrap up in a single novel.
I think that covers my thoughts on it. Anything I missed?