Take a photo of hiragana or katakana: a sign, a menu, a page from a picture book. Each word gets its romaji (and its meaning, for common words), so you can check your reading.
Basic free version. Kana are read right on your device, so it's free and private, but it works best on clear print, skips kanji and only knows the meaning of common words. A better paid version is available: it reads more accurately, handles kanji and gives the meaning of every word. Ask LUNO about it.
Romaji uses Hepburn spelling. Long vowels are written double (コーヒー = koohii). The particle は is shown as ha; in a sentence it's read wa.