I once heard these signs that your platy might be pregnant:
1) Generally round about the midsection, as in the sides.
2) Has a flattened off bottom belly.
3) Is, or has ever been, a "platy".
Short of isolation from all males, yes, your platy is pregnant.
And the thing is, after your platy gives birth, it's STILL pregnant.
And your other platy is probably pregnant too.
Livebearers man...