The test is to wait a MINIMUM OF 168 HOURS (this is very important) after the last time you sent them a text, and then send them a very universal conversation starter.
If they break the 24 hour reply rule again, then you have proven that they do not want to text you. That's because the chance of them coincidentally not having 15 seconds of time to reply to your text in 2 24 hour periods that are a week apart (assuming they have access and service to their phone) is less than 1 in a billion.
The only possible way this can be the case is if their phone is physically removed from their possession or destroyed. Although this is quite possible, if you keep in touch with the person through other means (facebook, myspace, or especially a face to face conversation) you can find out for sure.
Steve: Has it been 24 hours since you last texted her?
Peter: Yea, why?
Steve: Do the 7 day reply test, wait a week and send her a conversation starter if she doesn't reply 24 hours after that then her phone must be lost/stolen/broken or she ain't tryna text you