The differences between a male java finch and a female java finch can be difficult to identify if you don't have much experience with these birds. The easiest way to tell the difference is to compare a male and a female, so you can see the subtle variations between the two. Otherwise, your best bet is checking with a veterinarian, who can give a professional opinion and perform a conclusive DNA blood test.
The simplest way to identify a male java finch is by his behavior -- only the male will sing. Otherwise, you can look at a specimen's eyes and beak to determine male from female. The red rings around the java finch's eyes are a bolder hue on the male, as is his beak -- while both male and female have red and white beaks, the female's red coloring is paler by comparison. The male's beak is more swollen and pronounced than the female's, particularly on the underside, where it meets the neck. You may also observe the coloring of your birds' feathers -- across different mutations, the male typically has a darker-colored head than the female. If you are unable to determine whether your java finch is a male or female, take him to your avian vet, who can perform a simple DNA test.