Quintilian uses the phrase quomodo scribitur in Institutio Oratoria, Book I, 4, 11 (accessed via Perseus) in discussing doubling of vowels in a word. He is not using it as a direct question, but the phrase seems amenable for that purpose.
quaeret hoc etiam, quomodo duabus demum vocalibus in se ipsas coeundi
natura sit, cum consonantium nulla nisi alteram frangat. atqui littera
I sibi insidit, coniicit enim est ab illo iacit, et V, quomodo nunc
scribitur uulgus et seruus. sciat etiam Ciceroni placuisse aiio
Maiiamque geminata I scribere; quod si est, etiam iungetur ut
consonans.
The following excerpt from Suetonius about Augustus and his spelling habits based on pronunciation is fascinating. Note the definition of orthography in the first sentence. This suggests that the Romans noticed and commented on spelling errors.
(from the Loeb Library edition in public domain)
88 Orthographiam, id est formulam rationemque scribendi a grammaticis
institutam, non adeo custodit ac videtur eorum potius sequi opinionem,
qui perinde scribendum ac loquamur existiment. Nam quod saepe non
litteras modo sed syllabas aut permutat aut praeterit, communis
hominum error est. Nec ego id notarem, nisi mihi mirum videretur
tradidisse aliquos, legato eum consulari successorem dedisse ut rudi
et indocto, cuius manu "ixi" pro "ipsi" scriptum animadverterit.
Quotiens autem per notas scribit, B pro A, C pro B ac deinceps eadem
ratione sequentis litteras ponit; pro X autem duplex A.
This article about an embarrassing spelling mistake in an inscription (a modern S for a Greek sigma, Σ) on a new Classics building at Cambridge University in 2010 makes your point quite nicely about checking twice before using the chisel.
