Plastic isn't as heat tolerant, so you could subject the utensils to high heat (200°C should do it) and see if they start to melt. Whilst it's a destructive tes

Plastic isn't as heat tolerant, so you could subject the utensils to high heat (200°C should do it) and see if they start to melt. Whilst it's a destructive test, it's probably worth destroying and replacing plastic utensils as silicone ones are much better. Edit: apparently you can tell the difference by squeezing them too - silicone will feel rubbery.