Learn French: À ta place, j'ajouterais une garniture fraîche. - In your place, I would add a fresh garnish.

There isn't one! The phrase À ta place (In your place) acts as an implicit condition. It essentially means Si j'étais à ta place (If I were you). Because it sets up a hypothetical scenario without using si, the verb that follows must still be in the conditional tense (j'ajouterais).

How can I tell that j'ajouterais is the conditional and not the future tense?

In writing, the conditional always ends in -ais for the je form, while the future tense ends in -ai (j'ajouterai). In everyday spoken French, these two endings usually sound identical! You have to rely on context clues: the hypothetical setup of À ta place guarantees you are hearing the conditional.

Does une garniture just mean a decorative garnish like a sprig of parsley?

It can mean a visual garnish, but in French culinary terms, une garniture is usually much more substantial. It frequently refers to a side dish (like vegetables or potatoes served alongside meat) or a filling/topping (like the ingredients inside a crêpe or on top of a pizza).

Why is the adjective spelled fraîche here instead of frais?

Because garniture is a feminine noun, the adjective must agree with it. Frais (fresh) is an irregular adjective, and you are seeing its feminine form, fraîche.