@Dobby1sAFreeElf also, the evolve button when you look at a pokemon will still show a shadow if you haven't had the evolved form in that gender.
e.g. If you have only got a female Venusaur, then the evolve button on a male Ivysaur will show just the shadow, whereas on a female Ivysaur it will show the colour image.
For reference the pokemon which don't have gender are;
Magnemite/Magneton
Voltorb/Electrode
Staryu/Starmie
Ditto
Porygon/Porygon 2
Articuno, Zapdos & Moltres
Mew & Mewtwo
Raikou, Entei, Suicune
Lugia, Ho-Oh
Lunatone & Solrock
Baltoy/Claydoll
Beldum,/Metang/Metagross
Kyogre, Groudon, Rayquaza
Also, Some pokemon do have a gender but only come in one gender;
Male only include;
Nidoran (M) and evos; Tyrogue, Hitmonlee, Hitmonchan and Hitmontop; Tauros; Volbeat and Latios
Female only include;
Nidoran (F) and evos; Chansey and Blissey; Kangaskhan;
Smoochum and Jynx; Miltank and Latias
plus then you have Unown whose many forms display in the dex in the same way as genders.