Be especially aware of transposition if you're looking at music for clarinet, saxophone, trumpet, or horn.

If it's originally for flute or oboe, it's safe to play in the same key.

I've been familiar with transposition for a long time, mostly because I played in school bands and studied composition a little before I took up strings.

Violists steal from everyone, because there's so little viola solo repertoire written prior to the 20th century. Normally it's transcribed into concert pitch for easier reading. The Brahms E-flat major sonata I'm working on now was originally for clarinet, though in that case it was Brahms himself who rewrote it for viola in order to sell more copies. I've played one other piece that was originally for a transposing instrument: Schumann's Adagio and Allegro, originally for horn and piano. When transcribed in concert pitch, it's a good upper-intermediate-level piece for viola.

Bass clarinet is a standard orchestral instrument from the mid 19th century onward. MuseScore definitely has it, and the simple, mindless way to transpose would be to transcribe the part into a B-flat bass clarinet part in Musescore and then just copy/paste into a viola part.

I will note, however, that this bass clarinet part definitely goes below the viola's open C string, so you may want to play it an octave up (basically transpose as if for a regular clarinet). As written, it would match the cello's range more closely.

If you want to transpose directly: for any instrument, the sheet music will tell you what the transposition is. Transpose everything (including the key of the piece!) the same number of steps. The one thing that isn't stated directly is which octave to transpose to; for most instruments it's the shortest transposition from C, but there are exceptions. (E.g. the horn generally transposes downward.) The bass clarinet is one of those exceptions. It transposes to the octave below the clarinet; in orchestras the bass clarinet part is played by a clarinetist who doubles on the bass clarinet, so the idea is that the player reads the same fingering as the regular clarinet but sounds an octave lower. So for B-flat bass clarinet, the concert pitch is a major 9th (an octave plus a major 2nd) below the written pitch.

If it's not listed, sometimes you can figure out the transpositions from the key signature on the page. If the transposing instrument is paired with a non-transposing instrument, you'll see different key signatures.

Also, certain transpositions are so common as to be considered defaults for the instruments in question. For clarinet and trumpet it's B-flat, for horn it's F. Composers and arrangers should list the transposition anyway, but if they forget (which may happen with an inexperienced amateur composer or arranger), nine times out of ten it's the most common transposition for the instrument.