It depends on the insurer, in some cases yes your insurance can be made void if you do not declare the modifications however, there is the route of if it comes to claiming and the mods are not declared they can charge you the additional premium of what it would cost to add the modifications, this route could lead to paying more in the end and not being able to insure with this company again
If you read the small print in your insurance policy you will find a section that makes it very easy for them to get out of paying if you do not declare any modifications. They will declare the policy void in some cases, you could be effectively driving without insurance, not good! Telephone the insurer and declare everything, if you don't know the size of the alloys make a note of the tyre size, that should tell them what they need to know. If you have an accident it's too late.
Alloys might be an optional extra when the car was new but if you change the size of the wheel from a 14 inch to a 15 inch its a modification that might not have been available as an option when the car was new.
Telephone the insurer and declare everything, if you don't know the size of the alloys make a note of the tyre size, that should tell them what they need to know.