I haven't been ama-teur, ever, never.
Iambic pentame-ter, clever, better.
Based on your given emphasis, the first thing you can tell is that there are only 4 stressed syllables per line, so it cannot be pentameter (penta = 5); it is some form of tetrameter.
The second thing is that an iamb consists of exactly 2 syllables; the first is unstressed and the second stressed, "da-DA". (Some examples of iambic words: "confer", "inflate", "depress".) Since the rhythm clearly starts out with a three-syllable pattern, "da-DA-da", it also cannot be iambic. In fact, this particular stress pattern is called an amphibrach.
The fact that the lines deviate from strict adherence to the amphibrach does not significantly alter the intended meter; you simply give a slight pause or extension to the prior syllable to cover the missing element.
A sample of proper iambic pentameter would be
To strive, to seek, to find, and not to yield. ( (Alfred Tennyson, "Ulysses")
I suggest you read the linked page at Wikipedia for more details.
To answer your question directly about how to tell if something is iambic pentameter (or a reasonably acceptable variation):
- First, count the stressed syllables; if you can't come up with a way to reasonably get 5 stresses, it cannot be pentameter.
- Then listen to the rhythm of the stressed and unstressed; they should alternate, or very nearly so (sometimes an extra syllable just has to be thrown in or left out to make the grammar work). So the total syllable count per line should ideally be 10, but might be 9 or 11. If there's more than one unstressed syllable per stressed syllable, it cannot be iambic.
- Finally, the (vast) majority of lines should start with an unstressed syllable and end with a stressed one. If it routinely goes the other way around (Stressed-to-unstressed), it's a trochee, not an iamb.