How to know what temperature cling film is safe to heat to
It depends on the country since what is classed as safe differs although there are lots of common points.
My own summary from the below English, Irish and American food standards are

Unless a cling film is specifically marked as not safe for microwave use, it must be suitable for use in the microwave under EU law.

In general, cling film should not be used in an oven as it can melt and could contaminate the food.

Check with the manufacturer



While the Food Standards Authority in the UK does not have specific recommendations about microwaving with cling film, the Irish and American equivalents do. The Food Safety Authority of Ireland and The United States Department of Agriculture both advise that cling film should not touch food during cooking.
When using cling film, you can also check the packaging for any specific instructions, but interestingly you don’t have to choose cling film that is labelled ‘microwave safe’. Unless a cling film is specifically marked as not safe for microwave use, it must be suitable for use in the microwave under EU law.

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Can cling film be used in an oven?
In general, cling film should not be used in an oven as it can melt and could contaminate the food. However, there may be cling film available which can be used in an oven, so it is advisable to check the manufacturer’s instructions on the cling film before use.
Can cling film be used in a microwave?
Check the manufacturer’s instructions to see if it is safe for use in a microwave. If using in a microwave the cling film should not touch the food

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And finally

What containers and wraps are safe to use in the microwave oven?
Only use cookware that is specially manufactured for use in the microwave oven. Glass, ceramic containers, and all plastics that are safe to use usually will be labeled for microwave oven use.
SAFE TO USE:
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Wax paper, parchment paper, heavy plastic wrap. Do not allow plastic wrap to touch food; vent it to allow a steam escape.
NOT SAFE TO USE:
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Food cooked in any container or packaging that has warped or melted during heating.

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Related: Which plastic wrap is okay for oven use?