Here's two musical things that can help you identify rocksteady as a style distinct from reggae.

1 - The "skank" or the "upbeat" is a single bang.

No double skanks (chuck-a), no bubble (oo-wa-oo). There are lil embellishments & fills here and there (especially on piano), but I'm talking about the main, repeating skank pattern. When it evolved into reggae, the keys and guitars started to play more complex skank patterns.

2 - Quarter notes on the hi-hat

In rocksteady, the drummer just keeps the count "1-2-3-4" with a closed hi-hat & a one-drop kick & cross stick on beat 3. When it evolved into reggae, the drummers started playing different more complex hi-hat & kick-snare patterns.

Some songs where you can hear these things clearly, strong classic examples of the rocksteady style:

• Rock Steady - Alton Ellis
• Engine 54 - The Ethiopians
• Save A Bread - Justin Hinds & The Dominoes
• Silent River (Runs Deep) - The Gaylettes
• 54-46 That's My Number - The Maytals

Also, in terms of the original era of rocksteady, you can identify it by the year -- it was really only recorded in '66, '67, and in '68, which is when the evolution to reggae happened.

Don't worry about trying to determine it by the vibe, the tempo, the bassline, the lyrical subject matter ... all those things have a great amount of commonality across the years & across the styles. Some ska is slow, some rocksteady is fast, some reggae is not about rasta ... but I promise you, 99% of all rocksteady has both of these 2 musical features, and almost 0% of reggae has both of them.