SFR rental return question (GRM vs. Other Markets)

[Colton T.]

This might be a bit too in-depth for a forum post, but maybe someone can assist in answering it. 

Been buying real estate through-out my career, but not much experience in other investment industries. What are some ways to compare real estate holdings to other investment classes in order to help know when to sell / liquidate a portion of them to cash? 

I'm sure REITs have advanced ways of approaching this, and I realize it's not an easy question. But for a smaller investor (say <100 units), are there some good benchmarks to look at in comparison to other investment classes or the macro-economic environment as a whole? Let's assume there's no concern about a crash or major worldwide event, but rather just normal fluctuating markets. 

For example, if properties used to have a 8-10 GRM, but are now pushing 13-15 GRM, due to home prices appreciating faster than rents, that might appear like a sell signal, but only if there was better looking returns elsewhere. What are some ways to validate that and compare it to alternative investment returns?

Thanks.