Test: name the time (present, past, future) and the form (simple, continuous, perfect, perfect continuous), then combine them.
Present: works, is working, has worked, has been working.
Past: worked, was working, had worked, had been working.
Future: will work, will be working, will have worked, will have been working.
The twelve tenses at a glance
Recognition test: name the time (present, past, future) and the form (simple, continuous, perfect, perfect continuous), then combine the two labels. Had worked is past plus perfect, so it is past perfect.
Present simple: base form or an -s ending with no auxiliary. The shop opens at nine.
Present continuous: am/is/are before an -ing verb. The shop is opening.
Present perfect: have or has plus a past participle. The shop has opened.
Present perfect continuous: has been plus -ing. She has been opening parcels all morning.
Past simple: a finished event marked only by the past verb form, with no auxiliary. The shop opened early. Diagnose it by what is missing: neither a continuous auxiliary nor had. A bare past form can also be a participle, so the auxiliaries around it decide the tense. Compare The shop was opening (past continuous) with The shop had opened (past perfect). The two was/were clues separate past continuous from past simple and past perfect at a glance, and its -ing separates it from past perfect, which needs a participle instead. When two candidates sit this close, check the auxiliary before trusting the time word.
When the count itself gets fuzzy
The boundary is the count itself. Twelve is the standard count of time-plus-aspect combinations, but some references include extra forms such as used to, would always, and the future in the past, which raises the number, and some linguists count many more.
This matters when classifying a form like used to work. It names past habit, yet it has no slot in the twelve-item grid, so a strict test returns no answer. Treat it as an extra pattern alongside the main tenses, and note the same for would always and future-in-the-past forms.
The safer habit is to classify by what you see. Apply the time-plus-form test to the twelve core patterns first. If no pattern matches, say so plainly rather than forcing the form into the nearest tense, since authorities disagree on how the extras should be counted.